Create Checkout Session

Creates a Checkout Session. The response includes a hosted redirect_url where the customer completes payment, along with the computed totals and eligibility. Flex classifies the line items for HSA/FSA eligibility and, when a Letter of Medical Necessity is required, sets up the collection flow. The session's mode determines whether it results in a one-time payment, a subscription, an off-session charge against a saved payment method, or a setup-only flow. When the line items mix eligible and non-eligible products, a split cart is created so each portion can be paid with the appropriate payment method.