You can use domestic stamps for international postage, but you need to add 10%. So for a $3 international postage you need to use $3.30 of domestic stamps.
And for sure you can use whatever denominations you like to make that $3.30. Lots of people use three $1.10 stamps, for example.
You can certainly use domestic stamps on international mail, though AP regulations require that ten percent has to be added to pay GST, so you would need to put $3.30 worth of domestic stamps on an postcard.
As it sounds like you’ve already discovered…various staff will tell you various things.
I’ve literally pulled out this guide to show staff before…and then had to go through the calculations with them to show them I’ve used the right amount. It’s especially fun math if you’ve used some international and some domestic!!
Happy combining! I can sit for ages playing with various stamp sizes, orientations, themes, colours and values…trying to get them to look ‘perfect’. I quite enjoy it!