Sorry to hear John. BS they don't have a grace period so people with bulk legitimate stamps can use them prior.

I recently saw a post, on a PCF thread about shipping, about listings on a marketplace site I'd never heard of before. For sale were discounted postage stamps in bulk (say $100 worth for $60). Never heard of or seen counterfeit postage but unsurprisingly there were multiple comments warning the poster they might be fake.

My guess is the change is related to counterfeit postage. By limiting its use to only smaller/cheaper envelopes they're decreasing the amount of $ USPS is being taken for per instance so if they catch the same % of counterfeit postage, they'd be saving themselves more money.

And how are you supposed to mail any large envelope without going to the post office?
Regarding this, only way I know of is if you have a printer. You can use USPS.com (painful user interface) or my preference pirateship.com and print postage from home and they give you a good 5-20% off depending on the type of postage. You can also schedule package pickups on USPS.com if needed.