Which payment method does not relate to the primary functionality of Stripe Billing?

Stripe Billing primarily focuses on managing recurring payments and subscriptions, which includes functionalities related to subscription management, invoicing, and one-time product sales. While subscription management involves setting up and handling regular payments for services or products, invoicing capability directly relates to generating bills for customers, whether for subscriptions or one-time charges.

One-time product sales can also be processed using Stripe, as it allows businesses to sell products that don't rely on a subscription model.

Expense tracking, however, does not align with the core functionalities of Stripe Billing. Expense tracking is typically concerned with managing and monitoring outgoing finances rather than processing customer payments or managing subscriptions. Stripe Billing is designed to facilitate the billing processes associated with receiving payments, not with tracking expenses. Hence, it is clear that expense tracking is the choice that does not relate to the primary functionality of Stripe Billing.