The X-Cart checkout process is a bit outdated I feel given the fact that more and more systems out there are aiming at 1 step checkout process.
I believe version 5.x is going to solve this issue but in the meantime Padraic from
www.ryandesignstudio.com has already solved this problem quite efficiently, you may want to contact him. I believe his username on this forum is "balinor".
Good luck!