questions re sending federal tax return [first time filing]

Bit of background: I'm a non-resident alien that got my tax return prepared by employer-sponsored 3rd party. I have received my tax return with instructions that I'm supposed to mail it. Tax return itself is pretty hefty (30+ pages).

questions:

• do I need to buy any special envelope (at the UPSP office?) would or something like https://www.target.com/p/clasp-envelopes-gold-4-ct-up-9in-x-12in-up-up-8482/-/A-14430532 be okay

• I have read that I should sent that with certified mail - should I get some special stamps for that or can I just come to the window and tell them what I want to do and pay?

• can I pay my taxes on IRS site before my tax return gets delivered? (after all the counting it looks like I need to pay some money)

Thank you for your help!