The Price field is a field in Airtable. If you would like to add up the total amount of line items, you can use a rollup field. To show formula fields in your form (rollup fields are considered formulas), you would have to enable Compute Mode option from the settings first.

The Stripe demo on our website shows an example of a multi-products checkout. Make sure to go through the Sample Airtable View at the bottom and read the fields' notes. It should help you understand how to structure your base.