You will never be treated oddly if you do this... unless you're already talking to the person.
Saying "excuse me" before asking for something is perfectly fine and natural, particularly in your example... though it would be more common if you were interrupting them in some way.
So, if the shopkeeper has their back to you or is chatting with another employee, saying "excuse me" to get their attention is very natural, even in American English.
If, however, you already have spoken with the shopkeeper you wouldn't continue to say this unless you lost their attention in the interim.
As to your question about Americans vs Canadians... there is a joke about Canadians being overly polite and Americans being very direct, if not rude by comparison. While these stereotypes certainly have some merit, that doesn't mean that everyone fits them. Being polite is rarely out of place, regardless of where you are.
It's also worth noting that in the US, you'll be asking for quantities in pounds not kilos. Some shops may be able to do both but they will be more comfortable with pounds and the shops will be priced by the pound. And your phrase is missing a word:
Excuse me, would you please give me one kilo of apples?
And I'd probably not use "give":
Excuse me, I'd like to buy two pounds of apples.
Excuse me, I'd like two pounds of apples.
Excuse me, may I please have two pounds of apples?
You can always ask for them by quantity, too.