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 the Save & Continue option from your form's settings first.

The Stripe demo on our website shows an example of a multi-product 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.