Bags NOT ALLOWED FOR DROP OFF 40 MINUTES before flight departs after online check in

Honestly feel like airlines have made losses during COVID are trying to recover this money the wrong way! So our flight was at 130PM. We checked in online and the Boarding pass clearly states that the boarding gate closes 10 minutes before the flight leaves. We got to the check in counter at 1249 and we were told we can not drop off our lagguage considering we had checked in online . We requested that we are allowed to enter the flight and our bags get on the next plane which was at 430pm at a cost. They refused, they preferred to charge us an extra R3,200 with a R200 penalty. This is a domestic flight I do not understand how getting to the airport 40 minutes the plane leaves after checking in online was late to drop off the bags? The lady stated they have a 5minutes grace period which they choose whether to exercise it or not. This does not even make sense. Honestly should consider a refund for this unnecessary cost!