If you're sending parcels, post them over the counter and get a lodgement scan (and receipt). If that is within your stated handling time, you will not get defects.

If sending letters, maybe message the buyer letting them know that eBay estimates are notorious for being wrong.

Here's one somebody posted a while ago -

G'day <buyers first name>,

Thank you for your very prompt payment. I posted your item this afternoon. Aust Post has been a bit slow lately getting items to <your state>, so your item may take longer to arrive than the estimate that eBay puts in the listing. It may well arrive quickly, but just letting you know that it could take longer than the estimate. The question you get asked about delivery time when you are leaving feedback is not a survey. eBay now automatically defect sellers if the answer is 'No'.

If you have any issues whatsoever, please don't hesitate to contact me by replying to this message, or selecting the "ask about using my item or ask seller a question" option, before leaving feedback. Thank you for buying from me!

Cheers,

If you get too many defects, eBay WILL restrict you or shut you down, so best to take away the opportunity as far as possible.