The easiest ways are by PayPal or Stripe. We recommend that you pay via the customized payment links on the main portal page. If you somehow can't access that, there is a generic, publicly-viewable page with the appropriate links.
Remember that both providers deduct service fees, so that we will receive and credit you for less than you type in to send. You'll find fee information on the portal's front page and financial page. While the fees may seem steep, they may actually be the best fees you'll find for online transfers of our volume. Your credit card company imposes similar fees, but you just don't see them because merchants jack up their prices to compensate! However, if you have a better suggestion for how to accept payment online, then we're all ears on the hcoop-discuss mailing list.
We will also accept physical payments, like mailed checks or international money orders or whatever you find necessary, though it is significantly more of a pain for us. If you feel that you can't pay with PayPal or Stripe, open a support ticket explaining exactly which payment method you want to use and why you can't use PayPal or Checkout. Many people refuse to use PayPal because of "horror stories" that they've heard, but it seems very rare to meet anyone who personally has experienced anything bad of this kind from PayPal. It really is quite a safe and convenient way of sending and receiving money.