When most of your customers pay by card, processing fees are not an occasional nuisance. They are a predictable selling cost created by the transaction.
A useful pricing estimate should account for the processor you actually use rather than copying another vendor's assumption.
Check both parts of your processor's fee
Many card-processing plans include a percentage of the transaction plus a fixed fee. Ignoring the fixed amount can matter, especially on lower-priced transactions.
Start by checking your processor's current terms. A simple conservative estimate is:
(amount being processed × percentage fee) + fixed transaction fee
If your processor applies its percentage to the full checkout amount, use the full amount it actually processes.
Remember that sales tax can affect the processed amount
Sales tax collected from the customer is generally money you collect and remit under the rules that apply to your business; it is not the same thing as business revenue. But a processor may still charge its percentage fee on the tax-inclusive checkout total.
For example, if a pre-tax sale is $40 and the applicable sales tax is 10%, the customer pays $44. If your processor calculates its percentage on the full charge, the fee calculation uses $44 rather than $40.
This is a pricing worksheet concept, not tax advice. Sales-tax obligations vary by jurisdiction and situation, so use the rules that apply to your business and consult a qualified tax professional when needed.
If not every sale is a card sale, use your actual card share
If 95% of your sales are typically paid by card, a simple model can estimate the card fee and then weight it by that share rather than pretending every transaction is identical. If your mix changes, update the assumption.
Do not confuse processing fees with other business costs
Payment processing is only one per-sale cost. Transaction bags, receipt paper, tissue, carry boxes, and similar packaging used because a sale occurred may also need a place in the calculation.
The bigger pricing framework also includes production costs, event expenses, recurring business expenses, and the amount you want the business to retain or build.
See How to Price Handmade Products for Festivals: What Your Price Has to Carry for the broader structure.
Use your own processor, not somebody else's shortcut
Competitors can tell you what shoppers see in the market. Another vendor's card-fee assumption cannot tell you what your processor charges you.
The free Know Your Number™ Pricing Workshop includes a place to estimate card-processing cost. Processing fees and sales-tax treatment are covered in Episode 6 of Festival Geeks.