I can't believe you'd wait in line at the post office with that many boxes...
Buy some of
these or some variant, I bought like 1000 labels 5 years ago and I've gone through half of those in my online selling shennanigans. Definitely pays for itself when you can just walk down to the post office and dump packages off. Although I think you can talk to them and get some baskets to bring in bulk orders of shipping.
And then get the free priority mail boxes from the post office, then buy and print your own labels with a.) click-n-ship or b.) paypal's shipping service. You don't have to do it per order, you can reach the shipping page
here. You actually save money if you do it online vs. at the post office, they give free delivery confirmation on paypal and a small discount.
Then you can print labels and ship 24/7. At least mine is open 24/7 for the most part ( no staff after 6pm or whatever ).
Unless I'm mistaken, you made it sound like you were showing up at the post office with 100+ boxes and paying for all of the shipping there.
Well I guess the other things you need are a small scale (needs to read ounces, you can do 13oz or less first-class packages on paypal. and it's like $1.50-3.00), packaging tape, and a printer. I happened to have all of those I guess.