If you are sure that you already had the Easy Savings Business plan active at the time of the transaction and that the 13cent were not for any other expense as per the pricelist (there are more services with exactly that price), then simply contact bunq support and ask them.
We cannot really help you here as community members I guess.
Also note that you should provide such information like the plan you are on upfront, otherwise we can only guess here as fellow customers.
You started the thread with a very generic question that got answered, then came up with more information afterwards - that your plan is only a savings account. That indeed changes the situation a bit and you might want to ask bunq about that if you are sure that you didn't miss anything else you haven't told us yet ;-)
We can't see your account or your invoice - perhaps you made a topup instead of a bank transfer, perhaps you did some transaction export etc. that all cost 13cent according to the pricelist for example.
