shipping templates amazon automation or manual fbm.

i had set 3-4 days for priority mail but find a lot of times, i cannot buy priority mail shipping with amazon, it forces me to use priority mail express, so i have to buy the label in shippo. Why is this the case ?

So i decided to create a priority mail amazon automation and let amazon set the times ?

Is this a good idea ? I fear that the amazon automation will set crazy delivery days like 1 week and drop my sales.