According to a DEP official I spoke to in person, the reason they didn't do it online was because the system doesn't have the functionality to factor in the credit when making a new purchase. It's a new system that an outside vendor set up for them.
Of course, the ability COULD be built in, but it would require time and extra money they'd have to pay to the vendor. Instead of going that route, the mail-in system was the best they could do for now.
Take it for what it's worth. Not the answer we want to hear, but I believe he was being honest.