A therapy session in my practice is compensated with €90, which can be paid either in cash on the day or invoiced for subsequent payment by bank transfer.

You receive an out-of-pocket bill based on the table of fees of alternative practitioners (Gebührenverzeichnis für Heilpraktiker, GebüH). You can present this bill to your insurance company and will receive, where applicable, a (partial) refund. Please contact your insurance provider regarding this process.