A lot of European sellers use those. Once they're taped up I find them very hard to open without tearing the tab, so not sure I've ever been able to re-use one.
For a single 45 or LP the envelope type are ok with 3 good stiffeners (and taped up - had a few sent without tape and arrive open)
re: soundswholesale - bought some 7" stiffeners and cruciform mailers from him recently - the stiffeners are ok, not the best but perfectly fine (guess the cardboard quality varies), but the cruciform mailers are awful, really flimsy.