Manofman - The problem is the way in which the prices are displayed.
Using your example route with FR out Fri back Sun - price on the "flight" page of FR shows:
£12.99 out / £6.99 back (£19.98 total). plus tax and fees goes up to £64.53
Next page - add in 1 bag, with online check in - price now £94.53
Pay using VISA - price now £104.53
So, you have to go through 4 pages to get your final cost, which is a differance of £40 higher than that shown on page 1.