from casper wiki: https://wiki.casperdns.com/index.php/CasperVend_2/API
format for the linkmessage is:
llMessageLinked(LINK_SET, -10, TRANSACTION_ID + ":" + PURCHASER_KEY + ":" + PURCHASER_NAME + ":" + RECIPIENT_NAME + ":" + AMOUNT_PAID + ":" + GIFT_CARD_USED + ":" + MODE + ":" + PRODUCT_ID + ":" + ITEM_NAME, RECIPIENT_KEY);



default
{


    link_message(integer sender_num, integer num, string message, key id)
    {

        llSay(0, llList2CSV([sender_num, num, message, id]));

// 0, -10, cf610accd9120e0828fde71fd69e39d32c08779e:18875f5d-0de5-4eb3-a527-361a69431d24:Trail Lauridsen:Trail Lauridsen:1:0:norm:10908129:DMS Moored Ambiance Soundpack plugin - (Thunder), 18875f5d-0de5-4eb3-a527-361a69431d24
        list data = llParseString2List(message, [":"], []);
//        llOwnerSay("name: "+ llList2String(data, 3));
//        llOwnerSay("raw: "+ llList2String(data, 8));
//        string invoicetemp=llList2String(data, 8);
//        list invoicedata = llCSV2List(invoicetemp);

        integer len = llGetListLength(data);
        integer i;
        for( i = 0; i < len; i++ )
        {
            string tempstring = llList2String(data, i);
            list templist = llCSV2List(tempstring);
            integer tlen = llGetListLength(templist);
            integer ti;
            for( ti = 0; ti < len; ti++ )
                {
                    llOwnerSay((string) i +" "+ (string)ti + ":" + llList2String(templist, ti));
                }

        }
//        llOwnerSay("product: "+ llList2String(invoicedata, 0));
//        llOwnerSay("key: "+ llList2String(invoicedata, 1));
//        llOwnerSay(llDumpList2String(data, "#"));

    }
}

output:
[12:32] 3) CasperTech DropBox v2.50: 0 0:b2727c46d862ded9cee4d436cc385dd23ef97abb
[12:32] 3) CasperTech DropBox v2.50: 0 1:
[12:32] 3) CasperTech DropBox v2.50: 0 2:
[12:32] 3) CasperTech DropBox v2.50: 0 3:
[12:32] 3) CasperTech DropBox v2.50: 0 4:
[12:32] 3) CasperTech DropBox v2.50: 0 5:
[12:32] 3) CasperTech DropBox v2.50: 0 6:
[12:32] 3) CasperTech DropBox v2.50: 0 7:
[12:32] 3) CasperTech DropBox v2.50: 0 8:
[12:32] 3) CasperTech DropBox v2.50: 1 0:18875f5d-0de5-4eb3-a527-361a69431d24
[12:32] 3) CasperTech DropBox v2.50: 1 1:
[12:32] 3) CasperTech DropBox v2.50: 1 2:
[12:32] 3) CasperTech DropBox v2.50: 1 3:
[12:32] 3) CasperTech DropBox v2.50: 1 4:
[12:32] 3) CasperTech DropBox v2.50: 1 5:
[12:32] 3) CasperTech DropBox v2.50: 1 6:
[12:32] 3) CasperTech DropBox v2.50: 1 7:
[12:32] 3) CasperTech DropBox v2.50: 1 8:
[12:32] 3) CasperTech DropBox v2.50: 2 0:Trail Lauridsen
[12:32] 3) CasperTech DropBox v2.50: 2 1:
[12:32] 3) CasperTech DropBox v2.50: 2 2:
[12:32] 3) CasperTech DropBox v2.50: 2 3:
[12:32] 3) CasperTech DropBox v2.50: 2 4:
[12:32] 3) CasperTech DropBox v2.50: 2 5:
[12:32] 3) CasperTech DropBox v2.50: 2 6:
[12:32] 3) CasperTech DropBox v2.50: 2 7:
[12:32] 3) CasperTech DropBox v2.50: 2 8:
[12:32] 3) CasperTech DropBox v2.50: 3 0:Trendy Broek
[12:32] 3) CasperTech DropBox v2.50: 3 1:
[12:32] 3) CasperTech DropBox v2.50: 3 2:
[12:32] 3) CasperTech DropBox v2.50: 3 3:
[12:32] 3) CasperTech DropBox v2.50: 3 4:
[12:32] 3) CasperTech DropBox v2.50: 3 5:
[12:32] 3) CasperTech DropBox v2.50: 3 6:
[12:32] 3) CasperTech DropBox v2.50: 3 7:
[12:32] 3) CasperTech DropBox v2.50: 3 8:
[12:32] 3) CasperTech DropBox v2.50: 4 0:1
[12:32] 3) CasperTech DropBox v2.50: 4 1:
[12:32] 3) CasperTech DropBox v2.50: 4 2:
[12:32] 3) CasperTech DropBox v2.50: 4 3:
[12:32] 3) CasperTech DropBox v2.50: 4 4:
[12:32] 3) CasperTech DropBox v2.50: 4 5:
[12:32] 3) CasperTech DropBox v2.50: 4 6:
[12:32] 3) CasperTech DropBox v2.50: 4 7:
[12:32] 3) CasperTech DropBox v2.50: 4 8:
[12:32] 3) CasperTech DropBox v2.50: 5 0:0
[12:32] 3) CasperTech DropBox v2.50: 5 1:
[12:32] 3) CasperTech DropBox v2.50: 5 2:
[12:32] 3) CasperTech DropBox v2.50: 5 3:
[12:32] 3) CasperTech DropBox v2.50: 5 4:
[12:32] 3) CasperTech DropBox v2.50: 5 5:
[12:32] 3) CasperTech DropBox v2.50: 5 6:
[12:32] 3) CasperTech DropBox v2.50: 5 7:
[12:32] 3) CasperTech DropBox v2.50: 5 8:
[12:32] 3) CasperTech DropBox v2.50: 6 0:norm
[12:32] 3) CasperTech DropBox v2.50: 6 1:
[12:32] 3) CasperTech DropBox v2.50: 6 2:
[12:32] 3) CasperTech DropBox v2.50: 6 3:
[12:32] 3) CasperTech DropBox v2.50: 6 4:
[12:32] 3) CasperTech DropBox v2.50: 6 5:
[12:32] 3) CasperTech DropBox v2.50: 6 6:
[12:32] 3) CasperTech DropBox v2.50: 6 7:
[12:32] 3) CasperTech DropBox v2.50: 6 8:
[12:32] 3) CasperTech DropBox v2.50: 7 0:10908126
[12:32] 3) CasperTech DropBox v2.50: 7 1:
[12:32] 3) CasperTech DropBox v2.50: 7 2:
[12:32] 3) CasperTech DropBox v2.50: 7 3:
[12:32] 3) CasperTech DropBox v2.50: 7 4:
[12:32] 3) CasperTech DropBox v2.50: 7 5:
[12:32] 3) CasperTech DropBox v2.50: 7 6:
[12:32] 3) CasperTech DropBox v2.50: 7 7:
[12:32] 3) CasperTech DropBox v2.50: 7 8:
[12:32] 3) CasperTech DropBox v2.50: 8 0:DMS Moored Ambiance Soundpack plugin - (Sunny)
[12:32] 3) CasperTech DropBox v2.50: 8 1:
[12:32] 3) CasperTech DropBox v2.50: 8 2:
[12:32] 3) CasperTech DropBox v2.50: 8 3:
[12:32] 3) CasperTech DropBox v2.50: 8 4:
[12:32] 3) CasperTech DropBox v2.50: 8 5:
[12:32] 3) CasperTech DropBox v2.50: 8 6:
[12:32] 3) CasperTech DropBox v2.50: 8 7:
[12:32] 3) CasperTech DropBox v2.50: 8 8: