Register gift certificate code as Transaction ID
Currently the used gift certificate code is saved on an order as an order variable. Sadly, when using more than one gift certificate on a single order, this value gets overwritten with the last used gift certificate code.
It would make sense to save the gift certificate code on the "Transaction ID" field of the "payment" entry instead.
On a related note, copying over the "message" field to the "Description" could also be useful (for example, when issuing gift certificates as refunds and adding a custom note to the gift cert).