I'm the opposite: I'm a cheap SOB, and I don't care how long it takes if it saves a buck or two. For your situation, it probably makes sense - and you get the boxes for free too. What just wrinkles my skirt are the idiots on eBay who insist on shipping EVERYTHING Priority, cost be damned. Like a 60 lb sewing machine as but one for instance. I'll contact the seller if I'm really interested in their item - most will change it and actually have NO idea of the cost differential. I've saved 50% or more on shipping just by calling them out on it. Had shipping cost on one item go from almost $100 to $45.Anything but Priority is horridly slow and our shipping program doesn't support the alternates very well. It has taken stuff 2 weeks to arrive that were sent to me Parcel Post or equivalent. The other side of that is you have to individually calculate each parcel. That adds time and complexity. For the money, it is impossible to beat Priority Flat Rate if you do apples to apples by transit time to get there. Most stuff is 2 days anywhere in the US. That is 2nd day air via UPS or FedEx and they won't even begin to touch that for 15 bucks. They won't even do that with a flat document envelope.
I order in 1000's of fasteners. I have a rough idea of what will fit in a medium, that is my order quantity. Gets here in 2 days @ 40 lbs. for the same 15-17 bucks. Makes your point but the rest is tough to beat.
I figure most everyone is like me, if they order something, they want it as fast as they can get it as cheaply as that can be done without dipping into the expedited shipping pool.
OTOH, small but heavy for size items cost about the same to ship Priority vs ground. I have 3 lightweight items for sale on eBay right now that would fit handily in the $15 box, but are shipping ground for half the price. If they were water pumps or something a bit heavier, they might as well go Priority as ground is gonna be the same or maybe slightly more! It makes zero sense to me, but that's the rate schedule. That only works for small stuff, big stuff needs to go ground.
My eBay listings always say "Shipped ground to save you money!" Haven't had any complaints yet.