Last week we got an article accepted into PRL (Physical Review Letters). As is standard, they afterwards sent out an email saying
To help defray editorial and production expenses, APS, a not-for-profit, society publisher, expects authors of manuscripts to pay appropriate publication charges; publication charges are contributions from authors' institutions to the cost of disseminating research results and should be regarded as an essential and proper part of their research budget. You are requested to make a payment of $765 toward the cost of disseminating your research results. However, the inability to pay this charge will not affect the publication process of your manuscript.
So they say they expect us to pay, but at the same time they say the publication process continues independently of whether this happens.
What is the 'proper'/'sensible' thing to do? What is commonly done?