I agreed to get the love letters but the print shop is closed. What do I do?

I can't remember if this is true for Planescape (long time since I played it) but in BG1 & 2, some of the shops only open during the day time. Maybe you need to wait for daylight then try again. Sorry if you have already done this.

I don't know if this is related to your problem, but if you irritate the owner of the print shop, he teleports you out. If you do this one time too many, I believe the print shop is permanently closed... (I think this was fixed in a patch, though)

Just pretend to be a jealous suitor instead.

what's the deal with his teleporting anyway? I was hoping that if I was a mage and I watched him carefully I could learn, but I don't gain anything.

Install a patch - that resolves the problem.
Only one thing you can get in Print Shop - Anarchists membership!