Welcome to the dev forums. It's actually quite simple to submit a web app -- here are the steps:
1. Click on 'Apps and Services' tab at the top of your dev portal account
2. In the main area on the far right click the orange button titled 'Add a New App'
3. In the 'Choose a Platform' dialog that pops up, select 'Mobile Web' (second option)
4. In the 'New Web App Submission' page fill out the required fields
5. Continue filling out all the required fields in all tabs ('Availability & Pricing', 'Description', etc)
6. In the last tab called 'App File(s)' you can choose to either provide a URL to your app on the web, or if you have already packaged all the required files into a .zip archive, you can specify it there and upload the ZIP file at this time -- this is done in the 'Select the type of web app you want to submit' section
7. After all the fields in all tabs are completed, click the 'Submit App' button to send it to Amazon testers for review
On the other hand, if you want to create an Android app (and submit it as a .apk file), you can also do that, but for that you will have to choose 'Android' in Step 3.
I hope it answers your question, however please let us know if you have any further questions.
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