I'm not sure I understand the first question. Gift certificates
can be used to pay for postage. They are as good as cash for any Forum purchase.
More than one certificate can be used on a single purchase but the shopping cart isn't set up for that, so it will take an email or phone call to arrange the payment.
A certificate can be used for more than one purchase. The remaning balance on the certificate automatically becomes a store credit after the certificate is used.
If I did not answer everything, please ask again. Thanks.