Moderator's note: Including a link to your website in your question risks seeing you labelled as a spammer (with the subsequent termination of your membership) but your question seems to be genuine enough though, so I'll offer you an answer.
The best rates for sending parcels are usually obtained by going through brokers. The most popular service for
domestic parcels among members here seems to be this one:
https://www.parcel2go.com/
(I use it myself)
For international parcels though this is one of the leading players:
https://www.parcelmonkey.co.uk/
Also check out this service
https://uk.interparcel.com/
and this one
https://www.parcelhero.com/
If you're operating as a trader it might be best to consider forgetting about insurance services anyway. The occasional cost of having to refund a customer for an item that gets lost or damaged in transit is likely to be far lower than the aggregate costs of adding insurance cover onto all of your consignments.