Dropbox receipt validity, After H1B Transfer , need new stamping?


Old petitioner visa expiry: Aug 2021
New petitioner visa expiry: Aug 2022
Planned India travel March 2021 and return US April 2021

I have a question about h1b drop box. I had paid my drop box fee on dec 21 2020 and I am currently waiting for a confirmation to book an appointment. Due to COVID I see on forums that the appointment scheduling is available to be seen only for 1 month.
Q1 - How many days is the fee valid for? Can I book an appointment later if dates are not currently available?

Q2 - Can I potentially travel in March 2021 with a valid i797 (aug 2022) also my existing visa expires in aug 2021?

Thank you for for response in advance

Usually the fee is valid for 1 year at least. For current receipts, they have extended it until end of next year ( Dec 2021 end)
As long as your visa is valid, you can travel in and out. No restrictions. You only need valid visa to enter. It does not have to be on the new employer. Read below