charging, or not, for postage for the (hopefully) 99%+ of people who don't need a replacement is something that you factor in to your costs, and tell people upfront so it factors into their decision. your call.

charging for subsequent postage for the (hopefully) <1% of people who have a problem and who need your love and attention is a really bad idea.

because - even ignoring the terrible PR from forcing someone to spend cash because of your product issues - you lose the best friend you have: a backer with a problem who's WILLING AND HAPPY to actually work through this problem with you and help you fix it.