I'm going to take the Logan Express from Boston Airport to Framingham but I couldn't find a way to pay when I am on board on the bus from the internet. Is there a farebox on the Express bus so that I can pay in cash?
You can pay in cash or card before you exit:
When you reach Framingham they will ask you to remain on the bus and let those who prepaid to get off first. You will then pay before exiting.