How do i return a sick mouse to petco?

So I got a mouse from petco yesterday to put with the 2 i already have and ive been keeping a close eye on her. She gets along well with the other 2 and she seems to be eating and drinking fine, but her breathing is rattly and she sneezes quite a bit, so im thinking she has a respiratory infection (i could be wrong) The store has a 14 day return policy but im curious if they would treat her and let me take her back, or if i would just have to get another mouse? And should i call first or just take her in and explain whats going on?