Dealing with Recurring Purchase Orders with Variable Pricing


I was wondering if anybody could point me in the right direction on this. We have situation where we have multiple purchase orders with many lines and releases setup to cover services which we use. Typically these are telecom services where the unit price can change from month to month based on usage or a price change. So at the moment, we do the following;

1) Setup the Purchase Order to reflect the contract / pricing terms at the start.
2) Match off the invoice if correct, rare!
3) If the invoice doesn't match the PO then
4) Update the PO
5) receipt
6) invoice

This can be quite troublesome. We put the PO for two reason, an indication of contract and for cashflow. Also, just to through into the mix these are all project related but not to jobs.

I have thought about, using the purchase variance account as the same account as the expense account. but this won't update the purchase and hence the cashflow.

The other option, is to create parts and link them to a price list and get vantage to update the purchase orders with the new prices, is this possible?

Also, sorry, has any body managed to do a process which automates the receipt entry. E.g. Have a po and invoice , I want to be able to enter the invoice and automatically generating the receipt ?

Lots of questions I know but see the woods for the trees.... I think