Postal eggs can be pretty hit and miss, you did better than me on my first load I did this year with ebay eggs - I think I got 4 out of 32 or something!

11 - there were a good 12 or 14 of those dead in shell too, some stopped mid incubation and most were infertile - it can just be one of those things.
I run my R-com suro, if I'm using it as an inci and not a hatcher, at about 38 dgrees as I know the thermometer isn't the most accurate, I also run it dry, so humidity was around 32% with the vent open which I only upped as others have said when the first ones start to pip to 58% when they start to hatch the humidity goes up on it's own anyway. Get rid of the black sponge thing in the bottom as I'm sure it harbours bugs as it's so difficult to clean!, just put a jay cloth on the bottom for hatching. My main inci is a manual old thing with a digital temp/humidity in it and that I run at around 37.6 - 38.1 - it's rather tempremental!, humidity is anywhere between 25% and 45% when I remember to chuck some water in! - and the last few lots of ebay eggs of different breeds and species and have had about 50 - 100% hatch, alot have had ruptured air sacs too - but it really is down to luck/how postie handles them/how they're packed/the quality and age of the parents/their diet/age of eggs/quality of eggs and any bugs that are on the shells or if they have been cleaned. Also if you do candle your eggs, it's a good idea to, so you can take out any duff eggs as you go.
So don't give up hope I know it is quite disheartening but postal eggs really are difficult so you've not done too badly - I have had a good few totally duff lots from ebay this year and they weren't cheap either!!
I think also with the lavender coloured birds you need to breed recessive birds to each other to keep the colour as it's a dilute so people do more inbreeding than normal with these colours, and there aren't many about, which can affect fertility and hatch rates hugely, orps aren't always the best layers either and withh all this terrible weather it has thrown alot of birds off laying so the eggs you got may have been old ones.