You can definitely mention that experience when you apply with Disney! It helps to be familiar with certain policies, understand how CMs (including third party CMs) are expected to interact with guests, and to have previous experience with Disney orientations like Traditions (which you would experience a shortened version of w/ third party retail). I've worked Merch at the parks as well as third party retail at Springs and there's some overlap for sure.
That being said, I don't think turning this position down would necessarily hurt your chances of working at the parks later in life. Especially during busier periods, it's pretty easy to walk right into casting and get hired basically same-day. I would say take the PT position if you want it, but don't feel pressured to do so solely to catch Disney's attention. IMO it's how you present yourself in an interview that matters more than your experience, at least w/ regards to "in costume" roles like merch/QSFB/attractions, etc.