MajorApi is a RESTful wrapper around QuickBooks. QuickBooks is notoriously difficult to work with. Yes, they do have some REST APIs you can use with their IPP platform, but for developers making custom QuickBooks integrations you're almost stuck using WebConnector.
WebConnector requires you to write your own SOAP server which is not fun. We've written a nice REST service on top of this, so all you have to do is download the WebConnector software from QuickBooks and install the .qwc file we provide, and you can start sending customers and invoices to QuickBooks in under 5 minutes.
Let us know if you have further questions. Thanks!