This is the job of pricelists. Once a pricelist is set on the customer, every new quotation for that customer uses its prices automatically.
- Enable Pricelists in Sales > Configuration > Settings.
- Go to Sales > Products > Pricelists and create a Wholesale pricelist and a Retail pricelist.
- Add rules to the Wholesale pricelist, such as a fixed price or a percentage discount per product or category, with a minimum quantity for the extra discount tier.
- On each supermarket contact, open the Sales and Purchase tab and set the Wholesale pricelist.
- Create a test quotation for a supermarket and for a shop and compare the unit prices.
If salespeople still need to override prices, review access rights so only managers can edit them, and use the margin report to watch the result.