The ExpandShare app is completely free to download and use from the Apple App Store.
Most iOS users will already have an Apple ID on their device from previous downloads, but if you don't have one or if you choose Create New Apple ID when you are installing ExpandShare, it will prompt you for a payment method, which is just Apple trying to create a full user record.
Select None at this stage, and then follow the remaining steps to validate your new account, with no payment information required.
Because Apple asks for payment method for those creating a new Apple ID, it seems like it is required or that the app costs something, but you can choose the None option to skip past that. Even if you choose to add payment information for your new Apple ID, you will not be billed for downloading the ExpandShare app, which will always be available for free.
Apple's Support Site has an article that answers this question in further detail, in case you are still having issues: https://discussions.apple.com/thread/254652712