Working in Canada for US Company - invoice and border q's

Hi!

I'm a theatre designer in Toronto, and I have been asked to design the costumes for a musical in Connecticut this summer. I previously had a J1 visa to intern at this same theatre company, but I have been back working and living in Canada for 9 months. I will be doing all of the actual designing in Canada, and they are paying for the design. The amount they are paying me is enough that I would like to take time off (approx 3 months) and oversee the production, including the training of their staff, the execution of the design, be there for the run and for the tear-down.

My question is, are there any restrictions for me just sending this American company an invoice for my designer's fee and them paying me by cheque? I don't have to have any special visas or trade sanctions, etc, etc?

Then, if they are not paying me for my time down there this summer, and I have proof of residence, continuing employment, a letter from the theatre company, maybe even a letter from my boyfriend, etc... I can just ask to enter the US as a business visitor, right?

I am worried that I am missing some detail and that I will do all my designs only to be turned back at the border. From what I can tell, I am perfectly within the law to visit the theatre in order to train their employees and oversee the production.

Any advice or answers would be greatly appreciated!