Merge Price means merging all the product prices in the selling cart into a single product. 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 user's group role "BundlePrice" is activated just like in the picture below.

I. Merge/SUM Price Through Sell Menu
Step 1. Right-click on an item and choose Merge/SUM price.

Step 2. Then do the payment as usual sales transactions

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.