The renter wants a strap kit, a brake controller, a dolly \u2014 whatever \u2014 when they show up. You do not need a price override for this, and you should not use one: an override replaces the whole rental price, and it does not apply to card bookings.
Use Charge card on the rental instead. It is built for exactly this.
\ud83d\udd0e Open the rental
Go to Rentals and find their booking.\ud83d\udcb3 Tap Charge card
This charges the card saved on file at booking \u2014 no need to ask the customer for it again.\ud83e\uddfe Pick Catalog item
Each line has a type. Choose Catalog item and select the accessory from the list \u2014 these are the add-ons you set up under Settings \u2192 Purchase items & add-ons, and the price fills in automatically. Choose Misc charge instead for a one-off that is not in your list, and type your own description and amount.\u2795 Add more lines if you need them
Use + Add another line for a second accessory. It all goes through as one charge.\ud83d\udcb0 Charge
The button shows the exact total, tax included. One tap and it is done \u2014 and it lands on the customer\u2019s receipt as its own itemised line, so it is clear what they paid for.
\ud83d\udca1 Pro Tip
Set your common accessories up once under Settings \u2192 Purchase items & add-ons \u2014 strap kits, brake controllers, dollies. Then adding one at pickup is two taps, the price is always right, and you can mark any of them tax-free if it is not a taxable sale in your state.
\ud83d\udca1 Pro Tip
This is the same Charge card button you use for damage, cleaning or fuel after a return \u2014 any extra charge on a rental goes through here.