Merge Price is a merging of prices on products in a single purchase, so all products would become one price only. Merge Price usually happens when a Customer wants to combine the product prices. Or in some cases, it could be a discount.
Before you Merge/SUM Price, make sure your role "BundlePrice" is activated just like the picture below.
I. Merge/SUM Price Through selling pages
Step 1. Right-click on an item - Choose Merge/SUM price.
Step 2. Then do the payment as usual
Step 3. Once the transaction is complete, you have created the invoice with the merged price
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For the record, you can also sell your products with Bundle Price.
You could also hide your Price with Price Visible Restriction
If you have delivery fees in your store, you could also Custom Price for Delivery Fees Product.