You can go into you're account and select "Business Policies," there you can create a shipping policy that's flat rate or flat rate with a surcharge for additional items etc. Then in the listing you can assign that policy to the listing.
Now towards coins I don't know the USPS policy, eBay has nothing to do with that other than making sure the shipping offered by eBay DEFAULT is compliant with USPS policies. For example there are certain things that can be shipped "Media Mail" and those not. eBay stays compliant with what the Postal Service says can be sent that way but that's not to say a seller doesn't ship or try whatever, collectible bottle caps using that service.
Envelope mail tend to be just for correspondence, paying bills by mail and not for shipping products such as coins, stamps, post cards, sports cards etc albeit people do it. The USPS did have "First Class Parcel" (or something like that) mail not long back but that's all been replaced with USPS Ground Advantage service. eBay as a business has to stay compliant with what the USPS offers in services in as far as eBay's DEFAULT shipping policies.
The postal service is DEFINATELY cracking down on what they consider "Products" vs other forms of mail. They are doing this as they are trying to at the very least not loose money as they have for ever so long, they've been told basically "Get to running in the green vs red or else." A big part of that has come about due to Amazon's own delivery mechanism as Amazon was by far in the way the USPS's biggest merchandise shipping customer. Atop that they want replace all delivery vehicles with eVehicles at a rather expensive price tag as you might imagine.
Now from what my clerk whom I've known for years told me not long ago is the USPS is ramping up inspection mechanisms to enforce compliance with shipping options. What USED to happen (and still does but will change) is merchandise using incorrect services either has it return to sender. With improperly weighed packages where postage be due, I guess some services now will automatically bill the difference. That said, what he told me is improper service usage the USPS will be seeking to HOLD such parcels UNTIL proper postage is paid PLUS a service fee for holding it. Items unclaimed be handled just as they are now. At least, he told me, that's what's being proposed...
I asked how does that happen, I mean I know Media Mail has been subject to inspection but he told me that new systems going into place that scan and this/that are more advanced that what's currently and been in place. I'd guesstimate that means metal detectors, perhaps high lumen projectors or X-Ray and AI computer automation, but that's just a guess. According to him the USPS is cracking down or trying to on a variety of issues from Opioids being sent in the mail to mail theft and much more.
