Many customers dispute invoices – often just to delay payment.
Sending an invoice the next day helps but also use the description field in your invoicing system to write a full account of what you did. That reduces the ability for someone to dispute a bill and lets them see all the things you did in the 2 hours.
It will make them feel better about paying your bill if they can see all of the things you did to complete the job and therefore what value they received.
In the description field just keep typing and make the descriptions as detailed as you can.