How can you pay for more storage

First off, you need to identify where you are looking for storage. If you are seeing a message that your iPhone storage is full, you cannot purchase more storage for the iPhone, that is fixed when it is made and you purchase it. You can delete content to make more room, but that won't generally be much. If you are getting a message that your iCloud storage is full, that you can increase, however understand that it does not increase the storage of your iPhone. See this for increasing iCloud Storage. Upgrade to iCloud+ - Apple Support You can also take a look at this support document to understand the difference between iPhone and iCloud storage. What's the difference between device storage and iCloud storage? - Apple Support

Be very careful.

You may purchase iCloud storage. You may not purchase additional iPhone storage.

What's the difference between device storage and iCloud storage? - Apple Support