Joan, do you sing all the parts yourself?
I sing all of the parts myself because I want to be sure that they're done the way I want them. If I got someone else to sing some of the parts for me, I would have to wonder about whether they would get them finished on time, and then I would have to double check to make sure that they had gotten the notes correctly. I would also have to pay them, which would mean increased prices on the final product.
Some of the high soprano notes (especially on classical pieces) can be a strain, but I believe that I'm still offering something useful at a friendly price.
For bass parts, I use technology to help me. I sing the bass parts higher than they really are, and then use a Pitch Shifter to shift the audio down to what the true notes are. The result is that the bass parts have a different sound where you can somewhat tell that they have been altered (it sounds sort of like a guy with a bad cold). But again, you can hear the notes clearly and people do find the tracks useful for learning bass parts.