Beginning January 18, 2017, PayPal Standard will not accept transactions with a purchase amount that includes a "comma" in it. For example, if there is an order for a thousand dollars, the thousands separator would show up as $1,000.00.
One option, and the best, is to simply turn off PayPal Standard and activate PayPal Express Checkout, which is newer code and also will create an account automatically for new customers using PPE. Instructions on this are available at the Zen Cart website.
You can find out more on the change that needs to be made by visitng the official Zen Cart forum.
If you need us to change the applicable file for you, then simply order this service and be sure to fill out our Module Installation form after you finish checkout and click the blue banner requesting more info from you.
IMPORTANT: You do NOT need this fix if you are switching to PayPal Express Checkout. If you need assistance setting up PayPal Express checkout, that can be ordered on our website instead of this service.