What's a good term to denote a fixed cost?

You could flip it, and say it's $25, plus $5 for each additional item.

Would overhead work for your purposes?

From Merriam-Webster:

over·head

1. business expenses (as rent, insurance, or heating) not chargeable to a particular part of the work or product
2. ceiling; especially : the ceiling of a ship's compartment
3. a stroke in a racket game made above head height : smash

It depends on the nature of it. For a flat fee, I'd say reorder:

Cost: $20 plus $5 per item

For a startup cost, use something like initial or base fee:

Cost: $5 per item plus $20 base

For a minimum item cost:

Cost: $5 per item plus $20 minimum

Can't think of a specific contract term - it's normally put under "handling" or "shipping and handling"