This one?  https://mark1.vendecommerce.com/preorders/products/mk1-store-credit-top-up 

Store credit top ups are a way for subscribers and mail order people to pay for multiple items aside for them at Mk1.  We also use it as an online payment method for items that are not individually listed in the Mk1 webstore. 

If we send you a link it will also come with a list of product aside for you at the counter and a total.

eg.   a customer has the following ready to ship

AMAZING SPIDER-MAN #8 $9.00
AMERICAN MONSTER #2   $9.00
ARCHIE #6  $9.00
ASTRO CITY #32 $9.00
AETHER AND EMPIRE #1 $9.00
AVENGERS STANDOFF WELCOME PLEASANT HILL #1 $10.00
Total $55.00 

One way for the customer to pay for the above items online would be for us to create six different web listings for those comics and send an email with six different web links. Our customer would need to click on each link and add them one by one to a Mk1 shopping cart.  The much easier way is to have one link where in this case you add 55 x $1 Mk1 dollar credits to a cart  We use this store credit to pay for the comics and send them on its way as per our customers standing instructions.  

You can pay exactly the amount expected each delivery, or if you like, an amount over and above the expected amount - our system will track your store credit and report any unspent amount to be used in a future purchase. (We will immediately refund a customer any unspent credit on request).