ARPA is MRR divided by the number of paying customers.
ARPA = MRR ÷ paying customers
ARPA moves when the mix of customers changes, not only when prices do: many small new customers lower it while MRR grows. Look at it per plan or segment.
Count companies, not billing accounts: two Stripe customers that are one company are one account.
sumwerk shows ARPA on the overview and per segment, and nets several Stripe customers of one company into one customer.
A restricted, read-only key is all it takes. The first import runs in a few minutes; every number you see opens into the customers and Stripe references behind it.