Most Startups billed their own, but you could look at Magento (OpenSource), Netsuite (Enterprise) or some kind of Wufoo.com form with Stripe.com integration (Simple).

Bill.com is fantastic.

As per your business, Xero.com or FreshBook would be the best cloud based applications to manage your financials as well as managing receivables efficiently.

With Freshbooks you can integrate payment methods also to give more reasons to your client for motivating them to pay faster, you can also communicate with them through Freshbooks and all your invoice history can be viewable to your clients.

Quickbooks accounting software helps small businesses organize their expenses, control cash flows, track receipts, create custom invoices with a pay now button for one click payments, and the ability to instantly create reports. Quickbooks allows you to take care of your financial business from any device, making it great for those of you who like to work from anywhere and on any device. Quickbooks online pricing ranges from $10-$28/month depending on your needs. The PRO 2017 desktop version is $199.95/month. Zoho is an accounting software platform designed specially for small businesses. Zoho helps automate financial tasks, freeing up time for you to focus your attention on growing your business. For those of you dealing with inventory, Zoho offers a tool to help manage it easily. Zoho Books includes a few other appealing features, including a tax management tool to help manage deductions and exemptions, the ability to run numerous businesses on one account, and support for multiple currencies and languages.
You can read more here: https://blog.proto.io/11-financial-apps-can-take-startup-next-level/
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath