XFreeStyle allows Admin to assign specific products to have a price field that the customer can enter an amount in, rather than normal Zen Cart pricing.
Use cases might be - you need to bill for a service or product where the fee or price is variable - such as - a shipping overage charge; maybe you need to bill a client for a Project Deposit; maybe you are a Charity soliciting variable donation amounts
Zen Cart fixed product pricing will not permit the flexibility required in these scenarios
Here is what the customer sees when paying for a product or service -
Products can be set to 'Product is Customer Enter Price: Yes' on the product page in admin.
And here is the Admin UI for assigning products to be XFreeStyle -