Needing ideas on how to fix a sales proposal in excel

Needing help on how to best build or update this spreadsheet for sale pitches. During holiday months we have different pricing tiers and money back depending on buy level and it varies between brands. I currently have it setup with the first column drop down for supplier and second column populates all the deal levels for that supplier. Once you click a deal all you need to do is enter the cases. Everything populates from there perfectly. The problem is say a person selects the 25-49 case level with a $9 back per case. If they mistakenly type in 20 cases it’ll still populate the $9 back. What would be the best approach to deal with multiple suppliers and deal levels.