The Walgreens App is compatible with the A500, I have it running on mine at the moment. They just didn't set it for Android API 11+ (honeycomb) when they loaded it to the market.
The workaround is to install it on an Android 2 device, if you know someone with an Android smartphone. Then use Astro file manager /Application Backup to back up the walgreens app and create the .apk file. It will be in the /mnt/sdcard/backup/apps/ folder.
Send the file to the A500, bluetooth, email or PC file transfer. You will need to enable install from file by checking unknown sources in in Settings / Applications on the A500.
You could also email Walgreens and let them know that customers with tablets want to use their app, so they can fix it in the market , but I wouldn't wait for that.