Invoices and receipts will automatically be sent out to your Billing Email address as the payment is processed. You can set your Billing Email under Settings > Account.
However, if you need an old invoice (we have all been there, its ok), you can find copies of all your invoices in Settings > Billing > Manage Billing
You will then be able to see all your billing details. Scroll to the bottom of the page to find your Invoice History where you will be able to download all your past invoices.
If you have any further questions on this, then let us know at support@getmayday.com