Spencer, as with any online purchase, the first thing to consider is security. My philosophy is that if the website or the app creator is not a well known business or creator, I don't use it- period. As far as websites go, mobile use versus computer use is pretty much one in the same. As for Apps, well, I would make sure the app developer is one in the same with the business you are trying to use. For a silly example, If you want to use an Ebay app and it is created by ImaGonnaSteal , obviously, don't use it. I guess my key point is be cautious. I am one of those that reads the reviews prior to making a purchase or downloading an app. If I still have a question, I either ask a friend or the developer or business.
I don't know if that really is any help, but it sounds like you are relatively new to online purchases and it can be a time saver if done cautiously - but it can also cost you a lot more if one just starts clicking away. Be safe.