View:
@inherits UmbracoViewPage<UmbracoSite.Models.PaymentFormModel>
@{
    Layout = null;
}

Зачекайте, будь ласка. Якщо нічого не відбулося, то для того щоб продовжити - натисніть зелену кнопку

<form method="POST" accept-charset="utf-8" action="https://www.liqpay.com/api/pay">
    <input type="hidden" name="public_key" value="@Model.PublicKey" />
    <input type="hidden" name="amount" value="@Model.Amount" />
    <input type="hidden" name="currency" value="@Model.Currency" />
    <input type="hidden" name="description" value="@Model.Description" />
    <input type="hidden" name="order_id" value="@Model.OrderId" />
    <input type="hidden" name="type" value="@Model.Type" />
@if (Model.Sandbox)
{
    <input type="hidden" name="sandbox" value="1" />
}
    <input type="hidden" name="language" value="en" />
    <input type="hidden" name="result_url" value="@Model.ResultUrl" />
    <input type="hidden" name="server_url" value="@Model.ServiceUrl" />
    <input type="hidden" name="signature" value="@Model.Signature" />
@if (Model.Type == "subsribe")
{
    <input type="hidden" name="subscribe_date_start" value="@Model.SubscribeDateStart" />
    <input type="hidden" name="subscribe_periodicity" value="month" />
}
    <input type="image" src="//static.liqpay.com/buttons/d1en.radius.png" name="btn_text" />
</form>

<script>
    document.getElementsByTagName("form")[0].submit();
</script>

Controller:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Web;
using System.Web.Mvc;
using MySql.Data.MySqlClient;
using umbraco;
using umbraco.BusinessLogic;
using umbraco.cms.businesslogic.packager;
using Umbraco.Core;
using Umbraco.Core.Logging;
using Umbraco.Web.Mvc;
using UmbracoSite.Models;
using System.Configuration;

namespace UmbracoSite.Controllers
{
    public class PaymentSurfaceController : SurfaceController
    {
        protected UmbracoLPMLDatabase Database = new UmbracoLPMLDatabase();

        [HttpPost]
        public ActionResult Donate(DonateFormModel model)
        {
            if (!model.Amount.HasValue)
            {
                return Content("Error. Amount cannot be null");
            }

            var orderId = Database.AddDonate(model);

            var description = "Добровільна пожертва у фонд випускників ліцею. ";
            if (!model.Subscribe)
            {
                var node = uQuery.GetNode(model.ProjectId);
                description += node.Name;
            }

            var paymentModel = new PaymentFormModel
            {
                Amount = model.Amount.Value.ToString(),
                Currency = "UAH",
                Description = description,
                OrderId = orderId,
                PublicKey = ConfigurationManager.AppSettings["publicKey"],
                Type = model.Subscribe ? "subscribe" : "donate",
                ServiceUrl = ConfigurationManager.AppSettings["callbackUrl"],
                ResultUrl = ConfigurationManager.AppSettings["resultUrl"] + "?id=" + orderId,
                Sandbox = ConfigurationManager.AppSettings["sandbox"] == "1",
                SubscribeDateStart = DateTime.UtcNow.AddHours(3).ToString("yyyy-MM-dd HH:mm:ss")
            };

            var signature = ConfigurationManager.AppSettings["privateKey"] + paymentModel;
            signature = CalculateSignature(signature);
            paymentModel.Signature = signature;

            return View("PaymentHidden", paymentModel);
        }

        [HttpPost]
        public ActionResult Callback(CallbackModel model, string public_key, int order_id, string transaction_id, string sender_phone)
        {
            try
            {
                if (model != null)
                {
                    model.PublicKey = public_key;
                    model.OrderId = order_id;
                    model.TransactionId = transaction_id;
                    model.SenderPhone = sender_phone;
                }
                else
                {
                    throw new Exception("Model is null");
                }

                var paymentId = Database.AddPayment(model);

                var signature = ConfigurationManager.AppSettings["privateKey"] + model;

                signature = CalculateSignature(signature);

                if (signature == model.Signature)
                {
                    var donate = Database.UpdateDonate(model.OrderId, true);

                    var amount = double.Parse(model.Amount);

                    if (donate.ProjectId > 0 && donate.DonateStatusChanged)
                    {
                        var node = uQuery.GetNode(donate.ProjectId);
                        var collected = 0;
                        int.TryParse(node.GetProperty("collected").Value, out collected);
                        collected += (int)amount;
                        node.SetProperty("collected", collected);

                        var donatesCount = 0;
                        int.TryParse(node.GetProperty("donatesCount").Value, out donatesCount);
                        donatesCount++;
                        node.SetProperty("donatesCount", donatesCount);

                        node.Publish(true);
                    }
                }

                return Content("PaymentId: " + paymentId);
            }
            catch (Exception ex)
            {
                LogHelper.Error(this.GetType(), "CALLBACK ERROR: " + model, ex);
                throw;
            }

        }

        private string CalculateSignature(string data)
        {
            var bytes = Encoding.UTF8.GetBytes(data);
            var sha1 = SHA1Managed.Create();
            bytes = sha1.ComputeHash(bytes);
            return Convert.ToBase64String(bytes);
        }
    }
}

Model:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace UmbracoSite.Models
{
    public class PaymentFormModel
    {
        public string PublicKey { get; set; }
        public string Amount { get; set; }
        public string Currency { get; set; }
        public int OrderId { get; set; }
        public string Type { get; set; }
        public string Description { get; set; }
        public string ResultUrl { get; set; }
        public string ServiceUrl { get; set; }
        public string Signature { get; set; }
        public string SubscribeDateStart { get; set; }
        public bool Sandbox { get; set; }

        public override string ToString()
        {
            return Amount + Currency + PublicKey + OrderId + Type + Description + ResultUrl + ServiceUrl;
        }
    }
}