Hello,
Unfortunately, the encrypted string in the return URL feature is currently in beta mode and we will soon release the proper documentation and sample code for you to use it. For the time being, we would advise against is usage.
Since Beaster has already provided you with the format of the value that these variables may hold, here is a full description for each of them.
ap_totalamount: The final amount paid by the buyer, before transaction fee is subtracted.
ap_netamount*:The amount received after the fees are applied. It is calculated the following way: ap_netamount=ap_totalamount-ap_feeamount
ap_feeamount*: The fee amount that was applied to a transaction.
*Please note that ap_netamount & ap_feeamount are currenctly only being sent for a real transaction. Since no fees are being applied when Test Mode is ON, those variables will hold a value of 0.0