They are usually in packets you can open, with just a plastic snap. So I just open it and pull out all the plain ones and stick them in another packet.
It serves them right for expecting people to like beige.
Where are you from? I don't recall seeing packets of undies in a bag that snaps shut, except for perhaps novelty or speciality boxers.
THIS IS MY UNDERWEAR:
The horrid stripes must be removed and replaced with more purple. The bags are either snap or stickied down and can be opened and resealed.