Will my time working for the USPS during my break in service count for time in service?

The short answer is: No. If you get a federal civil service job after you hang up your uniform then both your military service and your USPS time will count towards your Federal Employee Retirement System (FERS) pension. Keep a copy of your final SF-50 from the USPS that shows your entry and departure dates and your DD 214 to document your military service dates. But you’ll need to be retirement eligible to get that pension. There’s a lot of different rules and eligibility requirements. OPM.gov is actually very informative. Good luck and thank you for serving.

No. Although your military service can count towards retirement eligibility for civil service (i.e. civilian government employees) it doesn't work the other way around. Next time you have a break in service join the Active Reserve (Army Reserve or National Guard) so you can still get time in service credit for pay and earn points toward retirement.