Being such as small amount if customs had opening it they would not have just sent it on, they would have confiscated it. If it had been a larger amount they may (and that's a big 'may') replace with an inert substance and do a controlled delivery, for 1 gram though that's never going to happen. To be honest it sounds like the vendor packaged it like that in the first place. Was still good that you took precautions though, not sure if the whole 'return to sender' thing really makes a difference if LE do come knocking but it adds to your plausible deniability I guess.