Seems like $ would be inviting confusion, since it's followed by numbers that can look kind-of like a dollar amount:
19vcWM6EEbQHVdN2W8NXv9ySgsPjbZ6gU3$64.20.45.38
means send $64.20 to my bitcoin address ?
Hyphen or underscore would be less confusing, but I think period would work really nicely:
19vcWM6EEbQHVdN2W8NXv9ySgsPjbZ6gU3.64.20.45.38
19vcWM6EEbQHVdN2W8NXv9ySgsPjbZ6gU3.skypaint.com
The advantage of that last one is it is a valid URL; if a user pasted that into Firefox's URL bar or searched for it in Google the website owner could display a helpful web page explaining how to pay.