Additional charge or deduct price, drop downs, vlookups

Hi and many thanks in advance.

Im trying to make a simple document where i have multiple products with a price.

If a customer isnt sure what to choose, id like to offer him/her the next product as well, but only show the additional charge.

I started a small workbook for it, looking for somebody who can show me what needs to be done next.
My skills arent really in Excel sumif functions, so if anybody can give me a push in the right direction it would be very highly appreciated.

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If its unclear what i mean to do, please ask.

Again, many thanks in advance!