What's the weight of the item?
I think you're getting confused with Royal Mail sizes. Under 25mm thick is classed as a large letter, and anything else is classed as a packet. There is no size limit to a packet (within reason). You don't need to use Standard Parcel service - only after a certain weight does it work out more cost effective.
Prices for 2nd class packets for the general public are:
You can get cheaper rates with a franking or business account - something to look at further down the road.
Of course, there is no tracking or proof of delivery with this, but for how cheap it is, it's worth the risk! I have very little trouble really.
Don't know what to suggest about liquids. They will be expensive to post as they'll be heavy!
There's no real alternative to Royal Mail - courier companies are great if you're sending heavier stuff, but can't compete with Royal Mail on lighter stuff. Generally under 2kg I'll use Royal Mail and over I'll go to courier.
All the best,