I'm actually really looking forward to it, and I was wondering: you find out your work location on check-in day, right? I'm also assuming it's non-negotiable-you work where they want you? Is picking up shifts elsewhere difficult? Do the lines of customers get pretty large? Are people fairly decent in how they treat you? Sorry about all the questions, lol, I actually have a few more
I did merch as did my older 2 DDs when they came through the program. Oldest was at MK, Main Street in the Confectionary. She could be assigned any of the shops/carts on that side, but opted to be kitchen trained, so she stayed put most of the time.
2nd DD was put at DTD in Once Upon a Toy (which she hated at first b/c it was "ordinary"

), but she grew to love it and they made her a money counter, so she had mostly late shifts. She extended her CP into the Bibbidi Bobbidi Boutique, then went seasonal at World of Disney before joining the Kim Possible opening team.
I mentioned during my CP interview that AK was my favorite park and that is where I wound up being placed. I was in Dinoland (yeah, let's talk about ugly costumes!!!

) and was also trained to be a money counter, so I didn't spend as much time onstage. Dinoland merch also handles the midway games, so there was a position that was devoted to managing that aspect of things, which is what I did most of the time.
I found out my location on Day 2. The first day was more about getting folks settled into housing (I was offsite, so skipped a bunch of that) and getting fingerprinted, etc. The 2nd day was when I got my location and I was SO happy to be at AK, mostly b/c of the shorter hours (I was a f/t grad student, plus homeschooling 2 kids, so early evenings were important to me). You are placed based on business needs and sometimes from clues in your interviews, so you could be anywhere. When I did my management internship, I was placed at a resort shop, which was a completely different animal from the parks.
Picking up shifts is generally pretty easy, and there are usually a bit to go around in merch. One of the beauties of it, really. Attractions folks are limited to their areas, but if you are trained on the computer system, not only can you pick up merch shifts anywhere, but often QSFB shifts as well (to run the registers). When sporting events are in town, there are opportunities at WWoS, too. My oldest has a different role now, but still sometimes picks up shifts at the Pirates shop just for fun. You will also find MANY CPs who want days off, so would like to give or trade a shift away, so just make yourself known as the go-to person for that.
There have been a few times we've had some long queues in my locations, but most people have just waited 120 minutes for Expedition Everest, so it's not the end of the world, especially if they are inside.

I can count on one hand the number of times I had a difficult Guest as a CP and every time, I was able to handle their situation on my own without needing a manager. It was all about empowerment and making magic and when Guests see that, it really makes them happy.