My 1st installation service statement read "past due" amount?

I had an installation on 2-7-08. It reads like this :
Installation---Amount $30.00
Service Fee--$25.00
I sent the m.order off 2-28-08 ,the service is a one day mailing address from where I live.
I then received an invoice dated 2-27-08
which had charges:
2-29-08 Balance forward---$55.00
3-01-08 March Invoice--$25.00---Balance $80.00
At the bottom of this statement were :
Current:$25.00 1-30 days Past Due- $55.00 Amount Due: $80.00
It says for questions ,please call.So ,I called to ask how I could be past due when I've only had the service for 21 days.I already lost 7days of service.The woman who handles custumer service .Said it was just how their computer software,has to have new customer information in.And that she knew I was not "past due".
I told her I was concerned with having a "past due"status on my bill.
She said "Well,if it's going to be a problem,they could come and pick up the unit."
This is a medical item a family member needs,which I also had to send for an

additional piece of equipment to go with it for $200.00.
I went hand delivered another $25.00 for next March 1st payment to be on time.But why should I accept my bill being of "past due" status when it is not.And it's as if, being the type of medical equipment you are dependant upon.They could so easily say,we'll just come and get it.I was not talking mean to this woman on the phone.I just wanted my status of past due taken off at some point.I do pay my bills on time! What do you think?

Wow, what an attitude by customer service!

I would dispute the fee in writing. And I would pay the invoice, except for the late fee.

I might also say, nicely, that there is no justification for the late fee and that you refuse to pay it, and you hope they will agree because you dislike using the small claims court even though it has always worked for you in prior disputes.

Then I would shop for another vendor.