More times than not, you can lower your discount rate just by calling.
The problem is then the processor adds the fees right back in by inflating interchange fees, adding new costs, etc.
You need to have a firm that looks at and analyzes every detail of everything on your statement and that has the proven expertise to spot any discrepancies.
Interchange fees make up 80-90% of your total processing cost.
Do they have an Interchange Optimization program to reduce those fees? If not, you will never see the lowest prices possible.
Are they making sure you are PCI compliant or addressing fraud issues?
Again, most of these companies will only negotiate a few low-hanging fees such as discount rates and nothing more.