Starting January 18, 2017 PayPal IPN / Standard will not be able to accept transactions with a purchase amount which includes a "comma" symbol. (ie: a comma such as a "thousands separator" in an amount such as $1,234.56)
There are a couple of ways to handle this issue.
1)  Change coding in a file so the comma symbol will not be an issue. Since PayPal Standard is "old" code, I highly recommend that you do option #2 instead of this option.  Here is a link to the Zen Cart forum that shows what to change based on the version of Zen Cart you have:  
If you would like for us to change the file for you, the cost is $20.
2)  Disable PayPal IPN/Standard, and install PayPal Express checkout.  This would be the "preferred" way to do things. The benefits of having PayPal Express checkout are that it is newer code than the "archaic" Paypal IPN/Standard code and it also will automagically create an account in your Zen Cart store when a new customer uses it to check out.
Instructions on how to set up PayPal Express checkout can be found here:
If you need us to set Paypal Express Checkout up for you, the charge is $35.