este es el error:
Parameter Error
Incorrect parameters were supplied, therefore this purchase cannot begin.
If you are attempting to purchase from a web site please contact the web site so that they can correct the error.
y asi lo envio
<FORM action=" " method=post target=_blank>
<input type='hidden' name='cart_order_id' value='23'>
<INPUT type='hidden' value='622748' name='sid'>
<INPUT type='hidden' value='1' name='quantity'>
<INPUT type='hidden' value='1' name='product_id'>
<INPUT type='hidden' value='Y' name='demo'>
<INPUT type=submit value="Comprar" name=submit>
</FORM>