[Repetition Compulsion]

I would imagine its a filesize thing. I think the stash is limited to smaller files but I'm not really sure. You could render to MP3 instead--you can do this from inside Reaper if you install the LAME encoder, I remember earlier you had mentioned rendering to .wav & then using an external conversion tool. Just make sure to stick with at least a 128 bit-rate, I usually do 320 and wind up with ~10mb files for a 5-6 minute song. Soundcloud really isn't a bad way to go, its how the majority of people here share tracks...I think posting wave files is more for when it needs some hyper-critical eyes or if you're sending it to someone else to master.

edit: I think you're good to go on the fluttering guitar volume, time to move forward. You're right about the vocals, I think the lead & backup vocals are mixed together fine, just a little loud in the mix. I don't know a whole lot (anything) about mixing vocals, but I think compression on those would help out for keeping it more even. You might also need to use a volume envelope on the lead vocals for the couple of times he growls...it gets significantly louder for some of those, with the envelope you could tame the volume by a few dB in those parts without affecting the rest of the track.

And yes, compression on the bass is normal (usually pretty necessary). Mine is compressed quite a bit, 12:1 ratio or something like that with a threshhold set for pretty constant 6db reduction, fast attack with medium release.