Spark supports Stripe and Braintree.
Spark supports monthly and yearly billing plans, as well as one-off charges.
Yes, Spark supports making one-off charges using Cashier's invoiceFor
method.
Yes, you can have both user and team plans at the same time.
Yes, you can limit team plans by team members. You can also limit plans by your own arbitrary conditions.
Yes, Spark supports collecting European VAT.
Spark is not designed to be integrated into existing applications. Spark is designed for new applications.