This caught a few people out on mobiles in particular where they might have accidentally made a purchase when browsing the shop. In response to your feedback I've added a confirmation message on these purchases. After recently adding confirmation to other purchases, I believe this now makes every single purchase have a confirmation message.
Another rare mistake we'd sometimes hear of was accidentally buying parts instead of engines (due to this area having tabs unlike all other purchases). To make things clearer the confirmation message literally states "Are you sure you want to buy X Part(s)? or "Are you sure you want to buy X Engine(s)?".
Hopefully this will make the shop easier to use and a nicer experience for everyone.