If you have automatic charges as a merchant (monthly fees you charge your customers), sometimes they are declined. Many of these could be circumvented.
A mechanism that allows you to check a charge in advance without actually charging the card. If you get a reply that the charge will not go through, you can alert the customer before the actual charge is applied to his card. It’s better for the client and you.
Offer them the following: they can get the product they chose for the regular price, or they can buy another product (either the same or another) and get a nice discount for both, but only if the second one is going to be a gift for someone. If it’s a consumable, you might want to have a permanent sticker on it saying “A gift to X from Y – brought to you by YOUR BRAND NAME” so they are more likely to gift it. If it’s not and it’s the same product (like two copies of the same book), then that is optional.
In any case, even if they don’t gift it, it will still help grow your transaction size. If they do, it will also help you spread the word about you and the product.