Hey there! I just redeemed an invitation for subeautique. However, there aren't any items in stock that I like. Can I wait with purchasing an item or will my invitation expire?

You can redeem it, and it goes under your "other currencies" until whenever you decide to use it :)

I always try to keep at least one redeemed, so i can always see whats in there, otherwise you cant see the items

Thank you again for the quick answer!

SBQ invites never expire. You can buy one item per invite you have under your currencies.