Sometimes, you can get away with not offering one. Customers are entitled to proof of purchase. Especially anything with a guarantee, it's in your best interests to give them one.
If you wish to run your business properly and accountable then you need to have both sales records (customer receipts/invoices) and purchase records (goods, assets, running costs etc).
__________________ Didley Squat now in stock. Going to be a FAST mover!
It is not a legal requirement to provide a receipt. But it really is best practice to do so.
Yes EMS1 it is best to, just get a folder and keep a copy of every invoice you send out. You may never need to use it but it may be useful at some stage.