Any time we've ever had weevels or ants get in the feed (or come with the feed)(we haven't had a mite issue ever) we always would put the bags in the deep freeze overnight like
@Kiki said. Only thing with doing this is the moisture it puts in the feed (it's prob already got some in there anyway, but freezing it seems to make it more moist). So we would keep the bag in there and feed straight from the freezer until it was at about 20 lbs and there was no longer a mold risk. I would definitely show the manager or store associate, that's not acceptable they sell it like that. Chewy.com is a great company (and decently priced) to order from.