you need to use the code like Saka sids on his post, and maybe using it in the post or submit button on your form , first reading the balance of the user and then compare the blance with the amount of your service, if the amount is more than the user balance redirect the user to the vodes purchase area.

Try to post here the name of the component , plugin etc and maybe we can help you

yes its posible.

you need change around the line 76 to 83 the simbol for euro ( €) to this for dollar ( $ )

example:
--------------------
<option value="50.00">50 &#8364; (500 <?php echo _VODE_VODES; ?>)</option>

to:
<option value="50.00">50 &#36; (500 <?php echo _VODE_VODES; ?>)</option>

--------------------

and you are ready to go