Jim, it is the same BS that I faced when I was working for someone else as a small company. The difference was it was not MY money it was the company money and I still got a paycheck. Now waiting for a month just because the customer is a big company just seems different.
We had some companies that always payed withing 10 days of the invoice. When they called and asked for anything they went to the front of the line.
There is an old saying "Fast pay makes fast friends" and companies that pay the bill promptly are much more appreciated than the folks that take 30 to 60 or 90 days and multiple statements mailed out.