Setting one up
Settings, then dues. A schedule names a tier, an amount, and an interval.
You can have more than one schedule per tier over time, but only one active at once. Sembr will not let you create a second active schedule that duplicates one you already have, because a member matching two prices is a bug that only shows up as an angry email months later.
What renewals do
Sembr works out who is due, raises the invoice, and sends it. You do not run anything.
When a dues invoice is paid, the member's expiry date moves forward by the term that invoice bought. Not by a calendar month from today, and not by whenever somebody got round to processing it. By the term.
That matters if the payment is late. A member whose year ran to March 31 and who pays on April 20 gets a year from March 31, not a year from April 20. They do not lose three weeks because they were slow, and they do not gain a free three weeks either.
Changing the price
Change the schedule. Existing unpaid invoices keep the price they were raised at.
That is the honest behaviour: an invoice you already sent someone is a document you already sent them, and quietly re-pricing it after the fact would be indefensible. If you genuinely need the new price on it, void the old invoice and raise a new one, which leaves both facts on the record.
Members who joined mid-year
Set their expiry when you create them, or when you import them. The renewal engine reads the expiry date, so a member who joined in September and whose year ends in September is invoiced in September, without you keeping a separate list.
Proration
There is none. A dues invoice buys a term and the term is the term.
We would rather say that plainly than ship a prorating engine that produces amounts nobody can explain to a treasurer. If you need to charge somebody a part year, raise an invoice for the amount you actually want.
What if a member changes tier
Their next renewal prices at the new tier's schedule. Sembr does not retroactively adjust the invoice they already paid, and it does not issue a difference. If you want to charge or refund the gap, do it explicitly as its own invoice or refund, where it can be seen.