I use PayPal postage and RM's SmartStamp for the majority of my postage, it is great for nearly everything bar recorded, special delivery or an item too big to go into the post box in which case it can only be done at the PO
I invested in a letter size guide, I picked up an official Royal Mail one from Ebay for around £10 I think as it is fantastic for working out what is letter, large letter or packet. Great if you post out lots like me, definitely worth the tenner I paid!
Also some scales are very handy
If your item is over 25mm thickness then it will be sent via packet, anything under is large letter format. Very little fits through the letter size (5mm thickness) other than um well, a straight forward letter! LOL
I hope that helps! LOL