We've covered a bunch of the now free PSTL tools. When Microsoft made the PSTL tools free they also made some other tools free including POP Over Receipt Tolerance.
This tool allows a receipt tolerance percentage for quantities. A setup window allows
the user to enter a tolerance percentage, that percentage is then used when a
Purchase Order is received. The highest quantity that can be received is the
original quantity ordered plus the percentage amount. I think that this is going native in GP 2013 but if you need it now you can get it.
These tools are available through your partner. I've always found them to be a pain to find so I'm putting up the link here.
The link to the Professional Service Tools Library AND the additional tools is https://mbs.microsoft.com/partnersource/downloads/releases/noam_pstl.htm