What are some alternatives?
When comparing FusionInvoice and WHMCS, you can also consider the following products
InvoicePlane - Free and open source invoicing application to host on your own server.
Blesta - Blesta is professional client management, billing, and support software.
Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
BoxBilling - Complete client management, billing & support software.
Zervant - Easy invoicing, expense management, reporting, and customer feedback.
HostBill - HostBill is a powerful and flexible, all-in-one automation, billing, client management and support platform for online businesses.