Two boxes?
Under Custom Destinations, you check United States and then fill in the dimensions (unless you were also using Calculated Shipping for domestic shipping) and the service you are using.
Do NOT add North and South America there or with other countries, because your US sales will be defaulted over to N&S America- at the higher International rates.
In some services, the domestic and US dimensions do not quite match and the US rates then do not appear. The easy fix is to increase the domestic parcel size by a single centimetre in any dimension. Without changing the domestic rate, this will allow the US rate to show.
And only use metric dimensions. Canada Post went metric in 1974 and any other measures are approximate guesswork.