Apple product engraving, if supplied by Apple, is only offered when ordered.
Personalised devices are engraved during manufacture in China - and are then shipped directly to the purchaser. Orders for non-personalised products are fulfilled from locally held stock - and will often, during these extraordinary times, be the quickest route to delivery of a new device. During the current global disruption of manufacture and shipping, due to Covid-19, devices shipping directly to the customer from China will likely take longer than usual.
Before making your decision, you may wish to consider that the value of a future “trade-in”, or private sale, will be significantly lower than that potentially achieved for an otherwise clean/undamaged device. Whilst future value may not be a concern to you, many customers have been subsequently disappointed in the value of their engraved device when seeking a future upgrade.
I hope this information and guidance proves to be helpful to you.