And I will take this opportunity to repeat some things that I have stated multiple times:
What mailing service types are recommended/permitted is not fully clear to sellers for several reasons:
(1) The Atomic Avenue shipping calculator does not tell sellers what shipping method it used to calculate the shipping reimbursement to the seller. Sometimes it is easy to tell, but sometimes the number that comes out is a real head-scratcher. More transparency there would help sellers, I think.
(2) I suspect that sometimes the AA shipping calculator isn't savvy about USPS insurance thresholds or charges. When shipping via Priority Mail, insurance is free for package value up to $50... but the charges can add up once you get up above that $50 mark. IMO exactly zero AA packages should be shipped via USPS Priority Mail without being fully insured.
(3) The Atomic Avenue help pages need an update. For example, the AA help pages refer to Parcel Post mailing when we were told some time ago that Parcel Post wasn't an option any more.
As a seller, I can tell you that sometimes (especially with orders that are large both in terms of high $$$ and physical size) the shipping calculator undershoots by a decent amount. In those cases, I typically elect to just eat the extra postage costs. It is poor form to go to AA and ask them to bug the customer for a postage adjustment after that customer thinks that they completed their full order... and three-way email conversations (me, AA, and the customer) take time to resolve themselves, and I make it a point to get my packages in the hands of the USPS by the next business day.
I will say, however, that once in a while AA substantially overshoots the shipping reimbursement. I just filled an order last night that cost me ~$20 to ship including full insurance... but the AA calculator billed the customer $36. I asked AA to make the adjustment at their end. But not all sellers do that, some will just pocket that difference and that's just uncool.