I just tested topping up with both €50 and $50, PayPal which is the maximum permitted.
I have in the past purchased 5000 crystals at a time which is $68.50, now I'm unable to. Yesterday when I was doing it, I could not purchase more than 3000 at one time, which is $41.10. I haven't seen a max transaction limit of $50 or €50 mentioned anywhere, where did you see that? Also, this still causes the same problem of multiple small transactions in a row flagging the credit card for suspicious activity.