Another “tip”… If your fingertip is dry, lightly moisten it, then try the sensor again.
My 7a fingerprint sensor with a Spigen glass screen protecter works fine about 95% of the time. When it doesn’t, quick lick of the fingertip and that usually does the trick.
Per the Liberapay site, one-time donations aren’t properly supported yet, but you can discontinue your donation immediately after the first payment.