Unless payment needs to be associated with automatic invoices, accepting money is as simple as sending an OKCash address link or displaying an OK QR code. This simple setup is within reach of any user and can fulfill the needs of a good range of clients. When done publicly, it is especially suitable for transparent donations and tips. You can even share OKCash via IRC or the different Social Networks like Reddit or twitter without even having a wallet installed.
There are many third party payment processing services that provide APIs; you don't need to store OKCash on your server and handle the security that this implies. Additionally, most of these APIs allow you to process invoices and exchange your OKCash into your local currency at competitive costs. Being Open Source you can use them to create new services as well.
If you don't use any third party APIs, you can integrate an OKCash node directly into your applications, allowing you to become your own bank and payment processor. With all the responsibilities that this implies, you can build amazing systems that process OKCash transactions however you would like.
OK creates a unique address for each transaction. So if you were to build a payment system associated with an invoice, you simply need to generate and monitor an OK address for each payment. You should never use the same address for more than one transaction.
Most security is handled by the protocol, eliminating the need for PCI compliance. Fraud prevention can be simplified down to monitoring a single variable: the confirmation score. Beyond that, keeping your OK secure is mainly a matter of securing your wallet and using HTTPS or other secure protocols to send payment requests to customers.
OKCash allows you to design new and creative online services that couldn't exist before because of financial limitations. This includes tipping systems, automated payment solutions, distributed crowd-funding services, time locked payment management, public asset tracking, low-trust escrow services, micro-payment channels, staking pools and much more.