To respond:
1. No high value stamps like this have the service printed on them anymore. Although the $19.99 stamp (note the stamp value is $19.99, not $19.95) is intended for Priority Mail Express Service, it can be used for payment of $19.99 in postage for any type of parcel being mailed at a US Post Office.
2. Yes, it does have a Scott Number (#4873).
3. No, the stamp is not sold out, at least not according to the USPS Store's website. You can still buy them at face value from SFS at the link below (adding the $1.30 shipping charge, though) or you might still be able to find them at some post offices:
https://store.usps.com/store/browse...r-stamp-rateMystic is typically overpriced for most modern issues and a $5 (or 25%) premium over face value (plus shipping) to buy this stamp from their inventory is nothing more than price gouging, as the stamp is not that hard to find.