Invoices

The only caveat, according to AT, is that you should keep an invoice from shops that have more than one activity, for example a newsagents where they sell various items with different IVA rates in case you feel the need to identify them individually in your IRS return. 

However, there are some situations in which it is beneficial to keep invoices, for example when purchasing a household equipment, electronic devices or jewellery, etc. In case of repairing, complaining, or replacing an item, the process becomes quicker if you have the invoice with you or at least a copy or a screen shot on your mobile, where the seller can quickly have access to the item code, date of purchase (in case of warranties), etc. But this is only necessary to assist with after sales problems and is not a requirement of the tax authorities.