All of Invoice Simple's applications (mobile and web) allow you to email your invoices or estimates directly within the app without the need to go another tab or complete any additional steps.
Once you've created your invoice and it's ready to go, you just need to tap on the built-in Send button located in the lower right corner (blue circle with a paper plane) and select the Email option.
After that you'll see the recipient's email address and a box to add a brief message to your customer which will be sent with the invoice. You could also tap on the + icon to add another recipient from your contact list. When you're done, simply press the Send button one more time to email the invoice or estimate.
You can also send invoices and estimates through your email app of choice by using the Share feature. Check out this other article for more information. Now you've learned two ways of sending your invoices and estimates via email!