Hello! I understand you’re having trouble signing into the USPS 2Day service, and if you manage to log in, it keeps asking you to sign in again. Let's work through this together.

First, can you let me know if you're using the USPS app or the website for logging in? Additionally, have you tried any troubleshooting steps so far, like clearing your browser cache or cookies? These steps can often help resolve sign-in issues.

Let's explore these potential fixes step-by-step:

1. Clear Cache and Cookies: If you're using a browser on your iPhone, it might help to clear the cache and cookies. This can often resolve any login issues related to stored data. You can do this by going to your browser settings and finding the privacy or data management section.

2. Close Background Tabs: Sometimes, having too many tabs open can cause problems. Closing other tabs might help your browser manage the session better.

3. Restart Your Device: A simple restart of your iPhone can sometimes resolve underlying issues that may not be immediately visible.

4. Check Network Connection: Ensure that you have a stable internet connection. If you’re on Wi-Fi, consider switching to mobile data, or vice versa, to see if that resolves the issue.

If you've tried these steps and you're still facing the same issue, please let me know, and we can look into further options, including checking for any account-related problems.