Understanding old British money – pounds, shillings and pence

I'll confess I still don't understand. Like the distinction between guineas and pounds and who you pay with each... Why!?!

Did this complicated accounting lead to systematic under or overpayment in particular situations? Merchants in the US pricing at $N.99 apparently fools a lot of people into misjudging the price. Were there breakpoints in this system that prices clustered around because of unexpected cognitive biases?