If possible, could you post a snapshot of the offer here? Hide all personal information before posting please.

It is highly doubtful it is for 3G speeds.  

The 3G plans are the government mandated starter plans

https://www.koodomobile.com/en/rate-plans?INTCMP=KMNew_NavMenu_Shop_Plans

My mistake.  Turns out that it was a marketing text by the monthly side of Koodo.  The plan was, in fact, 15GB at 4G, but switching to monthly takes away the flexibility to use USA add-ons, which are important to us.  The Koodo agent did, however, activate an offer on both our phones for 8GB additional data for 6 months at no additional charge.  It pays to ask!

Why would you be unable to keep or re-add the same US calling add-ons? 

Because the Monthly side of Koodo has no “add-ons”.  That’s a prepaid thing.  The 15GB is at 4g speeds.

I assume you mean the prepaid US roaming add-ons.

You are right, postpaid does not have these and use a daily Easy Roam charge instead which can become highly expensive, especially if you only need light daily use.