You will need to enter the May 15 date in the letter input.  If you want to change the April 18 due date for the first estimate, then you will have to do a custom letter.

I have tried to find out about e-pay dates, but I don't feel comfortable about the answer.

You won't be able to e-file extensions after April 18, so if you're used to e-filing & e-paying extensions you will have to prepare paper extensions and mail them and then go to the IRS website and e-pay there (much more time-consuming); or pay a month early.

I am hoping that a Lacerte Admin will hop on here and tell me I'm wrong.