You can use Clappia to create a custom invoicing app for your business. Here are a few reasons why Clappia should be your first choice for invoicing.

1. Fully customizable: You can customize each and every field to create an invoicing app of your choice. For example, you want drop-downs to select plans, select locations for different tax rates, add additional details etc.
2. Generate PDFs: You can create invoices in the desired format with all the headers, footers, logos, and other custom contents.  
3. Unique Ids: You can have unique numbers for your invoices. The format of these numbers is customizable as well.
4. Inter-connected apps: Of course an invoicing app is not a standalone app that you need. You can create several apps in Clappia and then connect them together for data flow to achieve the required purpose. For example, you can create a customer master and then select from a drop-down to pull all the details. Then you can create work orders corresponding to an invoice.