Just as a side note got a GTP myself recently so I am going through the joys of learning as well.

There are a few ways to get a fecal. If yours just passed one recently you can collect it and put it in a ziploc bag with a lightly moistened paper towel to keep it moist. They can also do a flush of the cloaca or stomach contents to get a sample so if you do not have a fecal sample, all is not lost. The pet store may have no reason to suspect parasites and for the most part they can stay in balance in a GTP for a long time. I don't want to assume parasites but it is wise to rule them out early to ensure the best health. If yours ends up having some do not despair at all, its not that serious so long as they don't get out of control. They can actually have a high load and not show any signs. Think of them as dogs and how common it is for them to get them

Really good to hear he is cruising around that is a great sign. The vet should be able to weigh him and give an idea from there. They will also be able to better check that lump. They should also check for mites, mouth rot, scale rot, etc in a routine exam so that should also give you a good starting point on their health.