What is the site I need to purchase the priority service?

After the Biometrics, I need to go online to purchase the priority service before sending out the whole package.

What is the site and what do I click to make sure I purchase the correct one?

As for the UPS, do I auto get a tracking or I need to pay for a extra thing?
What kind of envelope I should ask for to use to send all my documents?
Express?
I never use UPS before.   

I am applying for a settlement, spouse/wife visa.

Thanks!

Worldbridge:  https://www.visainfoservices.com/Pages/Welcome.aspx

Select the United States from the drop down menu and go to the "Purchase Additional Services" tab/link.

Choose either LA or NY (depending on where you're from) and you want Settlement Priority Service.

Purchase it on the day you send out your supporting documents, and follow the instructions.

What does this means?

You must write ‘Settlement Priority Service’ and your WorldBridge payment reference code on the outside of your visa application envelope.

I just write it on the UPS envelope and not on the visa application paper.

On the envelope you use to send off all your documents, you write:

"Settlement Priority Service" and whatever code you are provided with when you pay. 

(heads up, my code was looooooooooooong)

Thanks for telling me that.

1 more thing....
I only need to send 1 photo and it's ok to use the US passport side one.

Yep, US passport sized is fine.