Yes that's what I've done in the past, scanned the proof of posting and emailed it to them.
If your parcel will fit through the letter box then I think you are better off using the proof of posting rather than recorded. When things are returned to the sorting office because the customer isn't there to sign, I actually run into far more problems, this is because the customer either loses the card or can't be bothered to collect the item.
If it's a larger item of higher value then I will probably use recorded or special, because they are going to have to open the door to sign for it anyway.
Anything under £7 I definitely wouldn't bother with sending recorded because I look at it that my time is money and if I have to fill out paperwork in order to make a claim then that would probably cost £5 or more of my time to do so, so there's no point.
I know I could be more efficient, I could probably warrant paying for the collection service now, but I know it is only me and a few other ebay sellers keeping our local Post Office open so I keep going. If that Post Office closed I would definitely look into the collection service. It's quite funny because when the OAPs tut at me for having so many parcels the couple who run the Post Office tell me to take no notice because the OAPs have nothing better to do than collect their pension. They treat the ebay sellers like royalty and we get choccies and wine at Christmas.