Don't use what is really a consumer or start-up processing system such as PayPal, Venmo, Square, Stripe, etc. It looks very unprofessional.
I prepare all invoices in QBO and they sync to Bill & Pay, which is how all electronic payments handled. I can accept the big four credit cards or do an ACH, clients can set their account to just automatically pay on due date, or in some cases, payment plans. For example, I just signed a $12k consulting agreement and the payments are automatically initiated on the 1st of each month. I never have to think about it except when reconciling my bank accounts.
Biller Genie is another option but I prefer Bill & Pay. CPACharge, too, but I do not really care for their system. Another option is Bill.com but I did not care for how they could not directly import a PDF from QBO--in other words, I lost all branding on my invoices and it just became yet another generic invoice. Branding is a big deal to me, so that ruled out Bill.com for anything but AP.