I have tried several times to purchase 4 tickets to visit Les Milandes on Tuesday, May 24. Each time an automated message appears that the transaction cannot be completed. Any suggestions? Thanks!
Right on the website it says "pas de reservation nécessaire." which means reservations are not necessary. However, to answer your question, sometimes in France companies use credit card processors that accept only French credit cards. For example, on-line, IKEA won't process a US card, but I have no trouble in the store. I have many more examples like that where my US cards won't work online but they do in person. So, I wouldn't worry about it and just buy the tickets when you show up.
Otherwise, try changing computers and operating systems, clear your cookies, change credit cards. If none of these work, then just buy your tickets when you arrive.
When we were there 5 years ago, there was no issue of lines and such -- We were 4 and there were maybe 6 other people visiting.