No, if you get an invite and you've declined the invitation (returned your RSVP with a decline), then it's not "good etiquette" to buy a gift, regardless of "if you can" or not. Could you send a gift? Sure, but you could also just send a congratulations card, which WOULD be considered "good etiquette", since you were invited/notified of the event. With your logic, if I sent RSVP cards to the whole world they'd all be expected to buy me a gift IF THEY CAN lol. You're confusing what is OPTIONAL with what is considered GOOD ETIQUETTE. In addition, returning an RSVP with a decline means the couple can send invites to their back-up list. "If you can" DOESN'T come into the situation AT ALL, bc gifts are ALWAYS OPTIONAL at weddings. You attendance is all that is expected. This is what has been considered "good etiquette" for many decades.