When Integrating with 2Checkout using the following notes to configure the console at 2Checkout.

 

Set
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Account -> Site Management -> Approved URL = [nc site path]/[cart componentid]/2CheckoutReturn.nc
Account -> Site Management -> Direct Return = Give links back to my website || Header Redirect (Your URL)
Notifications (image link top right) -> Instant Notification Settings -> Order Created = [nc site path]/[cart componentid]/2CheckoutCallback.nc

Get (for use in the cart BE)
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Account Number (click person image top right)
Secret Word (Account -> Site Management -> Secret Word)