Depends what the format is. If they send you tunes then it’d be digital… so I wouldn’t then re-buy them when they come out. If vinyls an option I’d buy the vinyl.
If they kept back like 5-10 vinyls for friends and you received one you wouldn’t buy a new one would ya? If its about supporting the producer/label ask for their paypal and send them £10 or whatever.
Not “obligated”, but I would. I think it works itself out usually. I mean I’d only keep it if I liked it anyway and wanted to support. 9 times out of 10 it’s being sent for the sole reason of promotion and playing out. If it does get a release it will likely be many months after and it’ll probably be an updated version in some way.
Gets a bit more interesting if you’re someone that cuts to dub though and spend a decent chunk out of your own pocket.