help with SveaWebPay module URGENT!

I need urgent help with an ready made SveaWebPay module that another user did.

There are several payment options on Sveas server,
1. Card VISA/Mastercard - md5 problem.
2. Invoice - this works fine.
3. Direct payment bank- this is okey I think.

Error on number 1
"Exception: Specified and computed authentication codes mismatch!"

when tring to pay with card visa/mastercard
Sveas support says it is something with calculating the md5 from the request string

Can somone help fix this ? Price?