Amazon FBM & On-Time Delivery Rate

We have been selling on Amazon as FBM for over a year now and our sales have slowly been increasing month over month. However, over the last few months, we’ve noticed sales being spotty if not sluggish which is to be expected, but our on-time delivery rate (OTDR) metric has been below the 90% target in the past few months as well. The vast majority of our customers are buying with “Standard” shipping which is usually USPS Ground Advantage and we’re just getting peppered by USPS dropping the ball and delivery times are sometimes double what is advertised, if the package ever makes it at all.

So far, we’ve gotten one alert over the OTDR metric falling below 90% as well as premium ship options being removed at checkout (because of OTDR)..and we may get another one soon.

We’re looking for suggestions to avoid this scenario where shipments are lower and OTDR can vary much more dramatically with just a few bad deliveries. Do we just have to ship UPS ground every time and charge more for shipping (it’s almost double)? I don’t think that will go well with customers but would protect us more. Thoughts?