Merchant Security Updates: Learn how security upgrades may affect your integration at the 2016-2017 Merchant Security Roadmap Microsite
Payments Standard Updates: Beginning March 29th 2017, PayPal will offer a few enhancements that may require integration changes. Read more here:
Payments Standard Upgrade FAQ
I tried to set "custom" field with a value, I expected to see it in merchant email notification and/or in the order detail of my merchant account.
Unfortunately there is nothing, maybe a sandbox issue?
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