When you activated ProductCart, you were asked to provide the temporary user ID and password that were given to you at the time of purchase.
You can change the user ID and password at any time by selecting Update Master User from the Settings menu.
Please note:
Enter your existing user ID, the new user ID, your existing password, the new password twice to confirm your selection, and then click Submit.
For security reasons ProductCart will compare the two entries to the existing user ID and password currently in your store database, and return an error message if they don't match. ProductCart will also return an error message if the field values don’t follow the guidelines listed above (e.g. if you use letters in the user ID field). When the form is submitted successfully, ProductCart will display a confirmation message.
The technical reason why ProductCart requires the Control Panel User ID to be numerical is that a numerical User ID, along with an encrypted alphanumeric password, makes it harder for anyone to try to hack into a store and/or the store database by inputting a database query string. In other words, this approach reduces the chances for a SQL injection.