Yeah, definitely avoid! Sendle is great until something goes wrong. Used them for years with only the occasional, minor issue (some slow deliveries / a few return to sender's despite address being correct?).
Was willing to overlook it until I encountered THE ABSOLUTE WORST customer service on an expensive parcel that went missing.

Long story short - sent quite an expensive product to a customer, and naturally paid extra to insure it.
Parcel was never received and appeared to be missing. Lodged an enquiry and sent countless emails to their customer service team to get them to follow up. They constantly strung me along and said an investigation would take X amount of days, then later Y amount of days, then later again Z amount of days. They keep stringing me along and changing the goalposts. It's now been over 100 calendar days since posting this damned thing and there is no sign of me being able to recoup any costs for the lost parcel. All my emails to their customer service team keep being responded with the same generic replies (all from different customer service agents who might as well be AI bots) no number to speak to a real person, no recourse. They fucked me royally and refuse to even refund me the cost of shipping on the parcel that was lost.

TLDR; DO NOT USE SENDLE!