professional fundraiser

I raise money for various charities and I get paid hourly. The way our $/hour is figured out is based on a pay scale our emoloyer came up with. For instance we all have our own TSR # (telephone sales rep.) that gets printed on the mail we send out to our donors. So when it all gets mailed back and added up at the end of the week the more we collect the more per hour we can make. I'm wondering since we are hourly and not comission, is it legal for us to make $18.50 one week and, say. $11.50 the next week?? any ideas??