Neato, you're mixing in some oranges with the apple here.

It's not a requirement of etiquette that a person send a gift for a shower, or a graduation, or a housewarming, but it is a common rule of etiquette that a person who is invited to a wedding send a gift. One hopes that the person issuing the invitation knows you well enough so that your gift will have some meaning to them, but that's sometimes, unfortunately, not the case.

Just because you choose to ignore the common rules of etiquette, Miss Neato, does not mean they cease to exist. ;)

Now, having said that, I will admit that if I receive a wedding invitation from someone I don't know well, or whom I suspect doesn't really expect me to attend (cross-country) I would consider that "trawling for gifts" and I would send them a lovely card instead of a gift. But this poster asked what the etiquette was for his situation, and I told him the commonly accepted one. Everyone, including me, is of course free to modify it according to their own preferred level of boorishness. ;)