Did you pay €15 online for a "first" bag, or did you pay the correct online fee of €45 for the "second" bag?

I ask only because, if you take up the frequent flyer baggage allowance, this counts as the "first" bag. The "first" bag is always cheaper (€15 paid in advance, €30 at the airport, or, when you have FFP privileges - free).

Regardless of the "cost" of the first bag (€0, €15, €30), the second bag is always charged at a higher rate.

You should have paid €45 online for this bag (it costs €70 at the airport). I suspect that perhaps you instead paid just the €15....

So there may be a little more to this than first meets the eye.

Given your earlier personal experience, I would advocate that you always print your luggage receipt, and carry it together with your e-ticket. If you can anticipate potential trouble, it is always better to confront it on the day, rather than trying to recover the money later...