As has already been posted. Use one per fin if required. If still too short you'd need to go up to the next size strap. The difference between diff size straps is approx 3cm, the extenders are approx 1.5cm which is why you only get two, one for each strap. AFAIK you cannot buy them as a separate part, well I couldn't when I tried anyway, no idea why.
---------- Post added March 29th, 2015 at 10:57 AM ----------
Pop the buckle off the post and unscrews the buckle from the bungee. At the end of the bungee is a threaded end which screws onto the adaptors, it's quite well thought out.
---------- Post added March 29th, 2015 at 10:58 AM ----------
Sometimes they can be hard to unscrew and screw by hand, I use an adjustable crescent/wrench/spanner.
---------- Post added March 29th, 2015 at 11:04 AM ----------
Just realised why you maybe can't buy extenders, maybe they want you to buy a complete strap set.