OP should seek advice elsewhere then that vendor as others have mentioned.... many things to consider if really looking for aftermarket wheels and tires together.
What people have been referring is the diameter of the tire (aka height, and obviously proportional to rolling circumference) needs to be within expected specs of cars drivetrain / ECU within a range of error. This is more important with AWD vehicles that front and rear not so much different. Moreover, there is the appearance factor...like not having smaller looking tires (diameter or height) in the rear than in the front (unless that is intended to go with the mullet hairdo
).
In general for the model 3 sizes...the same diameter size tire would occur with a 40mm difference (235---> 275) in width while downsizing the width ratio by 5% (40-->35) to hover around a diameter of 26.5in (standard for model 3) and otherwise not going much out of 26 to 27in diameter size and not above 0.5" difference b/w front vs rear for AWD.
BUT since you are looking for new wheel / tire combo... other than the look and cost of the wheels, you should also seriously take into consideration:
-appropriate wheel width size for tire sizes being considered
-appropriate offset of the wheels
-as well as considering changing wheel width size upsizing / downsizing wheel diameter to 20s or 18s...there may be more options for tire sizes and desired effect.
Not to forget the obvious... wheel width change per inch (8.5 --> 9.5) is equivalent to 25mm in change of tire width for the same look of how the tire bulges out from the wheel lip (bulge is not desirable nor is excessive wheel lip, although each has their place along with mullets).
In general, if you are sticking to these size 19in wheels, would agree with pg62's recs above as best compromise ...should be using 235/40 and going slightly smaller in rear with 265/35 (or maybe 245/40 and 275/35)... these are good but not great looking options when all things are considered (also no clue about the offset since you did not mention further wheel details). With these wheel/tire sizes, the tire placement on the wheel width will look similar and it will be within the error range in overall tire diameter between front and rear.
If you do 245/40/19 front and 265/35/19 rear, your rear tires will be about half inch smaller than the front... "mullet compatible" look will be more obvious.
There are many options for wheels/tires combos for the model 3 (just search forum here), and if really wanting nice combo then pay attention to the details above and pics in posts from your search...all your choice, but make sure you get out of it what you want. The tire/wheel salesperson is likely not very familiar with model 3 and just trying to close the sale if truly had recommended those sizes (ie 275/45/19).
Happy shopping...(sorry for long post, just a little OCD)
