Small parcels, even sent regular 2nd class (no signature and no step by step tracking) receive a parcel I.D number which you can track on the Royal Mail website to confirm delivery... Handy as years ago this was not the case and I had a spat of eBay sales where buyers claimed they had not received an item and there was no way of disproving them so you end up losing the money and the item thus signed for became mandatory for your own safety.
My own rough guide is if the item is worth less than £20 it goes standard post, above that I used signed for and if over £100 I use special delivery. Worth noting that the cheapest option which is 2nd class standard still has £20 compensation for loss or damage and provided you keep all your receipts they do pay out, I've had to claim a few times.
*Also just realised you're sending blades which now have to be sent using Royal Mail's age verification service so that complicates matters. If you send them without accurately declaring the contents it doesn't matter which service you use as compensation for loss will be void.