Go to your local post office -- or call them -- and see what they say about lumpy packets with the flat rate padded envelope. I've had my local branch interpret the rules differently than info I found elsewhere, and it cost me extra postage... I was only shipping seeds, so the extra didn't add up to as much as if you end up needing to switch from flat rate envelopes to teh medium size boxes.
Looking at the medium flat rate box, I can slide a stack of 6 pairs of medium 370's or a stack of 3 pairs of medium thermo's into the box. There's enough extra space in the width of the box that L or XL gloves should fit similarly, although XL thermo gloves might start getting pretty tight. Looks like 4 of those "stacks" would fit along the length of the box.
So I'd estimate that the medium flat rate box should hold 24 pairs of 370 garden gloves, maybe only 18 pairs if all are XL. The same box should hold 12 pairs of thermo's, maybe only 9 pairs if they are XL. It's possible that more gloves could be stuffed in than I'm estimating, but I'd need 30 pairs of gloves on hand to test whether or not they'd really fit.
The small flat rate box will hold 6 size medium 370's, maybe only 5 if they're large or XL. It will hold only 2 size medium thermo gloves -- and larger thermo gloves would be hard to cram in at all (in addition to being bulkier, they are not anywhere near as flexible as the regular garden gloves) .
Although I didn't have the actual numbers in hand to try, I think the large flat rate box looks like it'll hold half again the numbers of the medium box -- that is, up to 36 pairs of Atlas 370's, or up to 12 pairs of thermo's plus 12 pairs of 370 garden gloves. Again, if you're ordering mostly XL, you need to adjust those numbers downward.
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That said, I think Sandy should stick with the estimated numbers she posted above, for people needing to pay postage in advance. It's good to have some margin, especially since I don't have enough gloves to really test the capacity of the boxes. At most, I'd say you could safely increase the max for a medium flat rate box (I was testing the thinner longer one that opens on the ends) to 18 pairs and the max for a large flat rate box to 30 pairs. (Those numbers have a margin of error of 6 pairs, possibly more, so I'm fairly comfortable suggesting them.)
The formula of 1 pair thermo's being like 2.5 pairs of regular gloves to ship should stay... For my medium size gloves, it was more like 1 to 2, but the larger sizes get really bulky, more so than the large 370's.
I would be happy to simply send an extra $5 or $6 in case my order needs to go into the next sized box or have an envelope added. That would be easy to refund in the box (yes I know you're not "supposed to" put cash in the mail, but I don't think $5 in a box of gloves is a big deal LOL).