Get infomation from invoice, and send it with url

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Hi I'm new to prestashop. 

 

I have read, in some online posts, that the only way to manipulate invoice, is through prestashop's core, is that right or wrong.

 

 

I'M making a module, that should extract shopping list from the invoice, after the credit card transaction is complete, and then send it, as a url, to a smsgateway, what class, functions, or hooks should I use ?

 

regards 

Anders from denmark.