Was it my immagination or do French banks not actually have any cash in them??!! LOL. They all look like business offices inside, with no teller windows (if you can even get past the security at the front door!). We learned the hard way not to take large bills at the ATM. We tried to cash 50 euro notes in four different banks in Paris only to be stared at blankly like we were from another planet. Finally at our last attempt someone took pity on us and pointed us to the post office where after 45 minutes in line we were able to get change. After that we made sure to take our money in 20's from the ATM. Wondered if this was just a French thing, or if we'd find the same set up at Italian, German, etc. banks.
You MAY have been going into "private" banks. They'll have an ATM you can use but they're primarily there for their private customers. I frequently receive 50 euro notes from ATM's all over Europe (since I make two or three large withdrawals vs. a lot of little ones) but have no problem breaking them when paying for anything from groceries to lunch.
This is a classical misinterpretation between cultures. Banking in France is different from the system of walk in and be served in the US. Each customer is assigned their own consultant with whom you have a working relationship regarding your entire account "private bank". Tellers will even contact your consultant for approval for transactions against your account. The walk in assumption is what caused the confusion it seems (and the blank stares) because in fact you clearly did not understand that all systems American have not been transported (thus the other planet).
You may also have mistaken the pity - what you got was a Frenchman who sent you to the worst option and were scolded indirectly - the referral to La Poste was a punishment of sorts (never known for good service, whether you live here or not).
Avoid the banks - try a Metro or RER station, Tabac, restaurant, busy Boulangerie (helps if you buy a baguette) - currency exchange will help as well.
Avez-vous la monnaie de cinquante Euros?
Thanks for the info. We were getting grumbles from metro station and grocery store about the 50's -- in each case we were purchasing at least 20euro of tickets/food and were given major attitude about accepting the large bills. In fact the grocery store didn't even want to make change for 25 euros, but she gave up in disgust when we started rummaging through our pockets and taking too long to give her the EXACT change! We had no problem with large bills at restaurants but after the hassles mentioned above figured we should get change, hence the hunt for a bank.