I know this really isn't a definitive answer, but at least prior to 3/3 on .bomb/PMUA, you lost the value of your eCert after you used it, even within the 24 hour window in my experience...however, I would sometimes find a Web Support agent who was willing and able to reapply it to a new flight within the 24 hour window with no penalty and retaining value. That said...it was a YMMV situation.
I know PMCO's eCerts held residual value and if you canceled you got that value back. Since they've used the PMCO system...does anyone know if the PMUA eCerts carry residual value and/or retain value if refunded.