Venmo, Square, and Paypal are two simple payment processing systems that we at CPI have used successfully and can recommend.
These payment processors work on multiple devices and can be used by website or phone app. The fees are reasonable and best of all the money is usually transferred to your account instantly or by the next day.
They offer buy now buttons or a shopping cart you can embed onto your website to receive payments. Invoices can be emailed with a payment link so that processing payments is extra simple.