Yes, DDR3L is compatible with DDR3 based systems. However, DDR3 is not forwards with DDR3L based systems, if the system requires DDR3L then installing DDR3 may not work. Do make sure all of the other requirements match.
To be honest, I wouldn't mix different brands of RAM which are the same size. You see, your system will use interleaving which is time sensitive (RAS & CAS) so you could encounter some hangs and crashes do to the slight differences in timing of the different units. This is independent of the voltage issue as the DDR3L will still be running at the standard 1.5 volts the DDR3 runs at.
I would just put in a matched pair of DDR3L modules instead here (replacing both modules with DDR3L).
I'm surprised you'er encountering problems locating the cheaper RAM. You may want to order it online from Kingston or other large memory house.