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Why are my custmers recieving an "Invalid Configuration" messeage?Asked by EasyAds on June 05, 2013 at 08:41 PM
I would suggest to review your Paypal API Keys, make sure the signature and api is correct.
Another possible reason is, since you were using Paypal Pro, please make sure you have accepted the merchant agreement in your PayPal Pro account after it has been approved.
Here's a step by step guide and information from paypal: http://wiki.shopify.com/PayPal#This_transaction_cannot_be_processed_due_to_an_invalid_merchant_configuration.