How can I make it so that customers can download my app on their ios device without using the app store?
Short answer: You can't.

You can't install native apps without Apple's App Store or a developer account's limited ability to distribute test software or an enterprise account's ability to distribute to it's registered devices.

Web apps are completely unrestricted, but have no means to "install". You have to instruct users to save the bookmark on their home screen.