re: Tax question about paying estimates

Short answer is yes.

The penalty might be avoided based on a lot of different things, or not even apply at all. Safe bet and easier to budget for, would be to send in the check on 9/15 with the estimated tax on the sale.

The "penalty" is the underestimation penalty, reported on Form 2210 (when required). You can take a look at those instructions if you want. The bottom line is that it is generally about 4% annual interest on the underpayment of the tax from the IRS' pay as you go system.

ETA: There would be no late payment penalty or late interest (in addition to the underpayment penalty above) that would apply unless you didn't have the tax paid by 4/15