Proration is billing only the unused or additional part of a period when a subscription changes in the middle of it.
Prorated amount ≈ (new price − old price) × remaining days ÷ days in the period
A customer who upgrades from €149 to €399 ten days before the period ends is billed about €83 for the rest of the period. That amount is real money and not recurring revenue.
Proration is the main reason MRR read from invoices jumps: the invoice after an upgrade is larger than any month of that customer.
sumwerk ignores proration lines for MRR and books the upgrade as one expansion on the day it happened. Proration lines are also left out of the daily check against Stripe's invoices.
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.