פדיון הבן without a כהן
According to תוספות, (second Tirutz Kiddushin 8a
ד"ה ר' כהנא) one may use a כהנת for
פדיון הבן. (even after she marries a ישראל)
Hence a פדיון הבן without a כהן
The Ch. Sofer writes If a כהן isn't available and the person who never had a Pidyon is in the Gevuros (the eighties) or a זריז who has no access to a כהן on the 30th day ( don't want to take chances of dying before having performed the Mitzvah)
They should use a כהנת without making the Bracha & make a תנאי that if no כהן will be found, then she shall be the redeemer.