Can I use one 8GB RAM module in place of two 4GB modules?
For the MacBook Pro 13-inch, Mid 2012, Apple only officially supports 8 GB RAM, 2 - 204-pin PC3-12800 (1600 MHz) DDR3 SO-DIMM; however, it can actually support 16 GB. So in theory you should be able to use a single 8 GB PC3-12800 (1600 MHz) DDR3 SO-DIMM.

You can purchase a single Crucial 8GB Single DDR3/DDR3L 1600 MT/s (PC3-12800) SODIMM 204-Pin Memory For Mac - CT8G3S160BM  from Amazon, currently $48.17 USD.

Or:

OWC Memory 8.0GB PC12800 DDR3L Module, currently $45.99 USD.

Crucial 8GB DDR3L-1600 SODIMM Memory for Mac from Crucial, currently $49.99 USD.

Have a look at the iFixit MacBook Pro 13" Unibody Mid 2012 RAM Replacement Guide.