No, the fondant cost you money and takes longer than the 5 minutes it takes to put a ribbon that the customer bought and delivered to you. A ribbon is a basic decoration and putting it on should be included in the standard price for a basic decoration cake. There's no way this should be an additional charge - unless she wanted you to go buy it.
You know, I'm not a "pro", I do this as a hobby. But I don't think it's ever taken me 1/2 hour to put ribbon on a 6 tier, let alone a 3 tier. It's going to take maybe 5 minutes. If I got a new bill for 1/2 hour decorating charge for ribbon that I bought and brought to the baker, I'd cancel. That's silly.
Why would you ice your cakes at the venue? All of my cakes are iced 2 days before an event and decorated the night before. Except in the case of a case that was just sticking satin ribbon and premade flowers, then I would ice and decorated the night before. Your cream cheese icing will be fine. You're either very quick and very good, or this is a disaster waiting to happen. I've never heard of anyone icing at the venue.
What brand of pan are you using? How are you preparing your pans (cake release, shortening & flour, collaring with wax paper, etc.)? What temp are you baking at? Are you overbaking? Why is your cake so much shorter than the pan?