Any game that requires the kit should automatically add the kit to the order as a separate line item or they should at least list it on the page so it can be added at the same time. Any good web store has the ability to do this. If theirs can't then they need spend some money to upgrade their site to do it. Or just make the default version for these games have the kit and bump the price.
This is more of a customer service issue. Nothing is more frustrating than spending $400 and not being able to use what you bought without having to go back and buy a separate $25 part.