What's a good term to denote a fixed cost?
Would overhead work for your purposes?
From Merriam-Webster:
over·head
- business expenses (as rent, insurance, or heating) not chargeable to a particular part of the work or product
- ceiling; especially : the ceiling of a ship's compartment
- a stroke in a racket game made above head height : smash
You could flip it, and say it's $25, plus $5 for each additional item.