Multiple shipping providers in nopCommerce version 3.20

Hi. I'm trying to understand how to use multiple shipping providers in nopCommerce version 3.20

So I configured USPS, UPS and FedEx

But "Estimate Shipping" only shows USPS... also on "SELECT SHIPPING METHOD" only USPS...

I would love to let my customers choose the mail carrier...

What am I missing?

You can use multiple "external" shipping plugins (e.g. carriers like UPS, FedEx, USPS, etc.)
You can only use one "internal" shipping plugin (e.g. Fixed, ByWeight, etc.)

When you set up the carriers, be sure you select appropriate shipping methods in their config pages (e.g. Priority, 2nd Day, etc.)

Check System > Log for any errors.

Have you acquired account/credentials for each carrier?   Have you had your carrier account approved for production (e.g.
call or send an email to the USPS, tell them you're using nopCommerce, and ask to have your profile updated for production)

First thanks a lot. I changed the options for UPS and it worked, also the password for FedEx was incorrect...

Now I'm trying figure out why should I want to use the "internal" shipping plugin...
Assuming I have USPS, UPS and FedEx setup - what is it good for exactly? (same prices for domestic and international?)

You don't need to use internal shipping rate calcs.  Some stores use them instead of carrier rate calcs, because they want to offer fixed rates, or rates based on weight, etc.  It is possible to use one internal rate calc and also use one or more carrier rate calcs, although it would be probably be rare.  Shameless plug... Shipping Director allows one to conditionally choose one or the other.  For example, use FedEx if cart weight is less than 150 lbs, otherwise use ByWeight