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Without further ado, I bring you the top 5 invoice applications for the Android OS (in no particular order).
PGM-Invoice: The advantage of the program is that it is very easy to use thanks to a user friendly interface. All features are clear and legible. Just set your company’s data, add products or services to sell, enter the customer and you're ready to go.

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InvoiceME: A simplistic Android invoicing application. Adequate for small business needs. Allows you to create your invoice and send it via e-mail or SMS. I have not personally used this Android invoice application but it has received mixed reviews on different smartphones. Please do the due diligence and research this application's performance on your specific smartphone before purchase.
SalesTrackr: Allows you to log hours, scan barcodes and send invoices directly from your Android smartphone. This program does what it was designed to do fairly effectively but the cost per invoice may discourage a lot of potential customers.
Invoice Pal: Another simplistic Android invoicing application. Create your invoices and send them via e-mail on your Android smartphone.
Google Docs: Create an invoice template in Google Docs in Excel or Word format. Fill in customer and product information on-the-go. Once the invoice is created simply e-mail, SMS or share it online all from within the Google Docs interface on your Android smartphone. This is currently my method of choice when it comes to creating invoices on my Android smartphone. While the Android applications listed above do offer some nice functionality and automate certain tasks I do not feel that they currently offer me enough over the Google Docs method to make it worthwhile to purchase the applications.