Most Luseta shampoos and conditioners come in two sizes, and several ranges are also sold as a matched set. Which to buy depends on how much hair you are washing and how often.
500 ml or 1 L
The 1 L bottle is the better value per millilitre in every range that offers it — roughly a third less per wash. It makes sense if you wash long or thick hair, if more than one person in the house uses the same range, or if it is going behind the chair in a salon.
The 500 ml is the right buy when you are trying a range for the first time, when hair is short or fine and a litre would sit open for a year, or when you rotate between two ranges. Shampoo that has been open a long time does not spoil, but fragrance and actives do fade.
The shampoo and conditioner sets
Biotin & Collagen, Keratin Smooth, Colour Brightening and Glossy Pearl are each sold as a matched shampoo-and-conditioner set as well as individually. Buying the set costs less than the two bottles separately, and the pair is formulated to work together — the conditioner is balanced to the shampoo's cleansing strength rather than being a generic one.
Sets are available in the same size tiers as the singles, so a 1 L set is the cheapest way to run a range as your everyday wash.
Where a mask fits
A mask replaces the conditioner once or twice a week rather than being used on top of it. If you are buying a set and a mask together, you will get through the conditioner more slowly — which is a reason to take the 500 ml conditioner and the 1 L shampoo rather than matching sizes.
Serums and oils
The repair serums and strengthening oils are small on purpose. They are applied to damp mid-lengths and ends, a few drops at a time, and a 100 ml bottle lasts months. There is no larger size because there does not need to be one.
A simple rule
Buy the 1 L of whatever you wash with most often, the 500 ml of anything you are still deciding about, and one mask. That covers a full routine without over-committing to a range you have not lived with yet.