I don't know whether you've actually looked at those cards in any detail, but if you do...

They have tick boxes to say what has happened to your parcel, one of which is "has been returned to the delivery office".

The others are:

"has been left with your neighbour at (address)"

and

"has been left in your designated safe place".

I am not sure how to go about informing the post office of which neighbour/safe place to leave things with/at - maybe there is some form on their website?

BUT:

It has been my experience that, yes, an annoying number of parcels go back to the delivery office, but where, when I have ordered something, I have had somewhere on the form to specify extra delivery instructions and have asked them to leave it somewhere safe if I am out, they have done so. (And in the case of Royal Mail, our postie now actually does this sometimes when I haven't asked.) I have also held parcels for nearby neighbours, so it may be worth coming to some arrangement with yours, if you have any who are in during the day.

A little bit of trust and community might go a long way in the absence of a perfect post office.