Rest assured, if you purchase any of the individual premium features, you will always have access to these features.
To purchase an individual premium feature, simply tap "Want just [premium feature]?" on the premium dialog that appears when accessing the premium feature. For example, you can tap "Import PDF" on the main screen of Squid, then tap "Want just PDF Import?" on the PDF Import premium dialog. This will take you to a screen that explains what PDF Import is and lets you purchase it.