Due to Costco's short max password length of 16 characters, I strongly recommend using a password manager to make a random password to satisfy all of these conditions below:
- Use between 8 and 16 characters
- Include at least one lowercase (a-z) and one uppercase letter (A-Z)
- Include at least one special character (e.g. !@#$&) - i.e., any symbol above the 0-9 keys)
- Does not contain blank spaces or the following special characters: < > ,
- Include at least one digit (0-9)
For the record, at least Costco.com has greatly improved after the summer of 2021 - before that, the site used to prohibit copying and pasting passwords, which prevented password managers from working properly. I believe the max password length was also longer before the summer of 2021 (either 20 or 32 characters), but I cannot confirm this since I never took a screenshot.