I produced a classic PayPal button with a drop down menu for pricing options. All my tests took me to PayPal.
My end user reported receiving "Problems with seller's site".
I created a new button and unchecked the hosted at PayPal option in step 2.
The problem seem to be solved, but I needed to know why both buttons worked for me and not the end user.
I found that recreating the button as not hosted at PayPal was not the correct solution.
The correct solution was to remove the special characters in one of the menu options, which read "Individual (Ltd. Qty).".
Lesson: do not use special characters in the labels for any pricing option.