When you send a file message, how long is the download period valid for the file?
Files are not removed as default.
But this can be changed (you need to create a support ticket to change it) Once you request this change, your files will be deleted after a retention period you define as described here (message_retention_hours):
If not removed by default
When uploading a file within 15MB
Are there any fluctuations in usage fees?
You need to talk to sales and agree the file upload size you need for your applications.