You should think of Amazon .com and .ca as 2 separate companies. Although your log-in information is shared, the order history and prime membership are not. You are not considered as a Prime member in the American site even if you have the Canadian membership. In other words, they will charge you extra shipping fee.
However, you can only order from the American site if you want it delivered to the USA address. You can order from either site if you want it delivered to the Canadian address but there will be an import duty.
For your Disney trip, to save money, here are my tips to use Amazon.COM:
1. Start a trial PRIME membership, free for one month, order as many as you want during the trip, then cancel it before charges start.
2. Deliver to the resorts that have NO business centres. The ones with business centres (ie, Grand Floridian, Contemporary) will be the one receiving them. You will have less business hours to pick them up and you have to pay a handling fee. For those without a business centre (ie, Wilderness Lodge), the parcels will be delivered to the front desk so you can pick up 24/7 without fee, unless you choose to tip the front desk staff.
3. Never ever order anything online to be shipped by Postal Services (USPS). Regular parcels will be delivered to the Disney central mailroom, instead of individual resorts. You will either wait one extra week to receive it, or have to drive to whichever that central mailroom place is.
4. Make sure you write, in the address field, either reservation number, or stay date, so they can match your name.