i personally never use it in through out my life therefore i cannot say anything about this, but i think it may be using for shopping in from online shops.
Bitpay is actually quite known for many merchants even big merchants are using bitpay to accept bitcoin so they are trusted for sure and many small to big merchants are using them. But i don't think ebay or amazon gonna consider bitpay to accept bitcoin actually they may not accept bitcoin for few more years in future.
yes and the reason is that bitpay is really trusted forum and you can do shopping using bitpay with any amount, as most of the people from different countries are using it.