I want to buy more space for pictures and videos

Not possible. There is no way to increase local storage of an iPhone.

You can purchase iCloud Storage if you want an get storage on an Apple Server somewhere. You can then use iCloud Photo Library to upload the Photos to the iCloud storage, and use the "Optimize" option in Settings->Photos to reduce the space taken up by those Photos on your iPhone.

See the "Save Space on your device" section in the following support document: click here-> Set up and use iCloud Photos - Apple Support