Lets see if I can unwind some of the confusion here. First PCIe is interface standard not what the device 'IS'.
In this case we are talking storage so your normal choices would be a traditional hard disk drive (HDD) or solid state drive (SSD). Here your system uses a blade type of SSD as the physical space is very limited.
The possible interfaces your system could have would be either SATA or PCIe depending on the age of your system. Here your system uses a custom PCIe 2.0 x4 interface (4 lanes).
So in a nutshell your system uses a PCIe SSD and your choices are limited as Apple designed their own blade SSD that while similar to the industry standard M.2 is not compatible!
For a frame of reference here is the only 3rd party that makes a compatible SSD for your system and their offerings: OWC - MacBook Pro with Retina Display 13" & 15" Late 2013, 2014, & 2015 SSD drives. Their drives are not as fast as the newer x4 Apple drives. So you might want to think if you need the larger drive now when you order your system or wait.