There are some links at the bottom of this post:
https://www.etsy.com/ca/teams/7654/etsy-international/discuss/17821085/page/6?post_id=355555017but they may be out of date.
1. Yes, there are border guards, & you have to pay a fee to cross when it is for a commercial purpose. You will need a passport or enhanced licence to cross. Some areas have long lines, as in hours long (so you may want to look into using one of the cross-border mailing services, if any are available in the area you are staying in - it may be faster & even cheaper)
2. How big is your average package? You can ship 100 grams (a bit less than 4 oz) within Canada for $1.80 CAD plus tax if they are under 2 cm thick, but there is no tracking. That would only require stamps, though, no labels - just photograph the stamped, addressed package before shipping as proof of shipping for Etsy purposes.
Shipping from Qc to the other end of Canada costs more in Canada than shipping it from the US, often, if you want tracking or it is over 2 cm thick.
Depending on where you are staying in Quebec & even within the larger cities, your nearest Canada Post outlet may not have many (if any) English speakers, as most are run through drug stores & variety stores. Proximity to Ontario is not necessarily a guarantee of more English being spoken, depending on the area. In other areas you will have zero problem finding people who understand you, though, so I expect this won't be a big issue.
You might want to look into the legal requirements for running your business for one month in a foreign country as a tourist - I am honestly not sure what this would fall under, & I have been away from Canadian immigration law too long to easily figure it out. Laws are the main reason I didn't do the same thing from the US back in 2013.
ETA - you might want to look for a local team in the area/s you plan on staying - they may be able to give you more specifics.
Have a great trip!