If your business deals in invoices and provides products or services to customers and wants to invoice them and get paid, you'll need to deal with accounts receivable. It's easy to start with a manual invoicing solution off the bat until you hammer out a product and then choose the right accounts receivable tool.
If you are a self-serve SaaS solution where someone can put in a credit card, or real with recurring billing you'll want to use a tool like Stripe Checkout, Zuora, Recurly, etc.
What is accounts receivable?
Accounts receivable is the money your business is owed resulting from your company providing goods and/or services on credit.
This is also something that Bill.com handles, as does quickbooks directly, allowing you to invoice and track receipt of payments. Receiving money and the A/R process tends to be a very domain specific need and there are many different products that cover various niches like subscriptions, e-commerce, service based, etc. This is one area where you will want to ask around to find the best one for your business.