On dot com they have extras like bases, po boxes and blocking shipping to Hawaii, Alaska / Protectorates. Unsure why there are these differences.
PO boxes are because some sellers use couriers like UPS who cannot deliver to USPS post boxes. (This may have changed recently.) And others block them because those sellers did and they are copy/pasting their ToS.
This is really dumb, because PO boxes are the safest addresses, way better than a box at the end of the driveway, handy for 'porch piracy'.
Ignoring the usual paranoia of Americans, residents outside the lower 48 states are charged more for shipping than mainland prices.
And while some sellers will decide to make the decision for their potential customers that the buyer can't afford it, some may also react to the higher fees paid when shipping is higher.
I sort sympathize with the APO addresses, since the demographic covered by troops is also the demographic with the highest delinquency and criminal behaviour rates- 15-35 and male. But like PO boxes, there is no real problem with delivery, even to overseas bases.
Canadian sellers tend to be more comfortable with selling internationally and since we have a smaller number of troopers, orders from a military address are less frequent than those from Nunavut.
The reason eBay introduced the GSP and now eIS was to encourage international shipping by xenophobic and paranoid sellers by offering greater protection than with domestic sales and freedom from the terrifying prospect of making a customs declaration.
TL/DR - Americans are scared of the internet and of selling online.
