I'm trying to balance a non-pofits books and have forgotten how to deal with incoming revenue. Do I need to have a invoice or receipt for payment that come from online?

Hello Trevor Lee, 

Thanks for contacting us in the Community. QuickBooks is a robust program that helps you to manage your accounting with ease. I'll be glad to share more info so you're on the right track with your work. 

In order to record your transactions the correct way, I recommend consulting an accounting specialist for expert advice. This ensures your books remain accurate while you continue to manage your work with peace of mind. You can also get in touch with a professional using this link here. Other accounting professionals are able to see this post in the Community and this will allow them to chime in. 

Let me know if this info helps. I'm here to steer you in the right direction.

If you have an open invoice that was sent to someone (for a product or service you provide) you can receive the payment against that invoice.  

If you receive payment from someone and you do not have an invoice in QBO you would want to record that incoming money against the correct incoming revenue account that pertains to.  For example, if someone donates to the non-profit or buys a membership (and you don't send out invoices for this), you want to capture that incoming money in the correct account.  There may be an account for Memberships or Fundraising etc. you just need to make sure you are selecting the correct account that the money pertains to so that you have an accurate capture of the incoming funds when creating an entry <-- this can be done using a Journal Entry or Sales receipt if you have set up the product/service.  

As long as you don't do both options (invoice the customer & accept payment AND do a journal entry for the incoming payment) you should be only capturing the payment once.