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NOTE: the script contains a table name prefix ‘#_’ which must be either changed or removed.

Magento < 1.5 use:
http://www.eliasinteractive.com/blog/magento-ecommerce-how-to-reset-all-test-order-information-and-set-unique-prefix-for-orders-invoices-shipments-and-credit-memos/

Magento >= 1.5 use:
http://www.myscienceisbetter.info/delete-test-orders-in-magento-1-5-x.html

Once you’ve deleted your test orders and publish your live website you might find you get the error PayPal gateway has rejected request. Payment has already been made for this InvoiceID (#10412: Duplicate invoice) when trying to checkout live orders with paypal. This is because your paypal account has already processed a test order with this order id once and will not accept a second order with the same order no.

Here you you have two options:

1) change the padding of invoice numbers so your new invoice numbers won’t conflict with the old deleted ones. You can do this by changing the setting in your database e.g.

update `eav_entity_type` set `increment_pad_length`=10 where `entity_type_code`=’order’;

2) you can change the starting invoice number to a number greater than the last test order number to avoid the conflict - see http://www.magentocommerce.com/boards/viewthread/10985/ for more information