Why a screen is not enough
Your monitor makes colour with light. A tin makes it with ink over a white base coat over steel. Those are different enough that a colour can look right on both and still not match.
Metallic areas are worse, because on metal a highlight is an actual reflection rather than a light grey. No screen shows you that.
What you get
The artwork printed on the material, in the finish you specified, at the size you specified. Registration is confirmed per format at this stage.
If your tin sits inside a printed carton, we proof both together, because the same Pantone on metal and on board will never look identical and you want to see them side by side rather than find out later.
What to check
Colour first, in daylight rather than under office lighting. Then type, especially anything small and anything near a corner or a shoulder where the draw has stretched it.
Then the boring one people skip: spelling, batch fields, and any legal text. It is the cheapest thing to fix at this stage and the most expensive to fix after.
Nothing runs without your approval
Volume does not start until you have signed the proof off. That is the point of it existing.
If something is wrong, that is what the step is for. Changing a plate is inconvenient. Changing fifty thousand printed tins is not.
Hold one before
you specify it.
Tell us what you're packing and we'll send the formats that suit it.