Monitor sales C++

#include <iostream>
//std::setprecision()
#include <iomanip>
// priceme.cp
using std::cout;
using std::cin;
using std::numeric_limits;
using std::streamsize;
using std::setprecision;
void options();
void Quantity();
double get_subtotal(int tm,int tg,int td,
            double mp, double gp,double dp);
void total_sales(double st);
void clear_error();
int main(){
      double
      monitor=159.99,
      getdone=179.99,
      delight=249.99;
      int q=0,option=0,mc=0,mg=0,md=0;
      char run='y';
      while(run=='y'){
            options();
            cin>>option;
                  if(!cin){
                        clear_error();
                        continue;
                  }
            switch(option){
            case 1:
            Quantity();
            cin>>q;
            if(!cin){
                  clear_error();
                  continue;
           }
                  mc+=q;
            break;
            case 2:
            Quantity();
            cin>>q;
            if(!cin){
                  clear_error();
                  continue;
           }
                   mg+=q;
            break;
              case 3:
            Quantity();
            cin>>q;
            if(!cin){
                  clear_error();
                  continue;
           }
                  md+=q;
            break;
            case 4:
            run='n';
            break;
            default:
                  clear_error();
            break;
            } // end switch
      } // end while
total_sales(get_subtotal(mc,mg,md,monitor,getdone,delight));
return 0;
}
double get_subtotal(int tm,int tg,int td,
            double mp, double gp,double dp)
{
      cout<<"\n\n\t\tTOTAL ORDER\n"
      <<tm<<" ICU Monitors @ $"<<(tm*mp)<<"\n"
      <<tg<<" Get'er done mon. @ $"<<(tg*gp)<<"\n"
      <<td<<" Gamer's delight @ $"<<(td*dp)<<"\n"
      <<"\n";
return ((tm*mp)+(tg*gp)+(td*dp));
}
void total_sales(double st)
{
      cout<<"\nTotal Cost of Monitors: $"
      << st
      <<"\nSales Tax (calculated at 6.5%): $"
      <<setprecision(5)
      <<((st*6.5)/100)
      <<"\nBalance Due: $"
      <<(st+(st*0.065))
      <<"\n";
}
void options()
{
      cout<<"\n\n\t\tMENU \n"
      <<"\n1. ICU Monitor $159.99"
      <<"\n2. Get'er Done! Monitor $179.99"
      <<"\n3. Gamer's Delight Monitor $249.99"
      <<"\n4. Checkout"
      <<"\nEnter the option (1-4): ";
}
void Quantity()
{
      cout<<"Enter quantity.\n";
}
void clear_error()
{
       cout<<"\nInvalid option\n" ;
       cin.clear();
       cin.ignore(
       numeric_limits<streamsize>::max(),
       '\n');
}