It was cranking very slowly bc of a weak battery so l brought it into pepboys to change the battery and now when l turn the ignition the engine starts then idles right back down to zero. There are no check engine lights and when the engine does stay on, idle is at 300 rpm. If they didnt use a reverse post battery would it have even started? Please advise and thank you to whoever can answer this problem for me.

If they truly charged the battery correctly, they should have run a test on it to make sure it was good. You may have a weak alternator not pushing enough current to recharge the battery. Take the vehicle back and tell them to test the entire charging system.

In my 28 years as a Master Tech I've never heard of a reverse post battery. Top post and side post are common. You have a problem with fuel pressure from what l can tell. I'd run a fuel pressure test and make sure you have around 42 pounds of pressure.

Could be a problem with the security system. They car might think someone tried to steal it when you charged it. Put the key in the driver door and lock all doors with the key in the door and then turn to the left 3-5 times, they doors will unlock 2 times into it.