Cabin baggage allowance
LCC are strict against the cabin baggage allowance, but now major airlines, flag carriers have strict conditions too.
Then, passengers on the same economy class flight does not mean that every passenger has the same condition for cabin baggage allowance. For example, Czech airlines have Economy Flex, Economy Plus and Economy Lite. Then, passengers of Flex and Plus can carry a small item and a cabin bag(carry on) but passenger of Lite can carry only 1 cabin bag.
Which means if you have a ticket for Flex, you can carry your handbag(purse) and cabin size trolley with you to the aircraft but if you have a Lite ticket, you have to put your handbag into your cabin size trolley.
Of course the fares for Flex and Lite are different and Flex is expensive than the Lite. Therefore the traveler would rather choose the cheaper one without acknowledging that there is a cabin baggage allowance. If you have a Lite ticket and have two items. You might have to pay for the check-in baggage to let you bring the carry on trolley. Then, usually fee for the check in item will be expensive than the difference between the two tickets.
The worse scenario is in case you have 3 items for Lite ticket. It can happen as you have a cabin size trolley, a backpack and a purse. Then, trolley and the backpack are full due to you are on the way back to home. If you cannot squeeze your purse into the trolley or the backpack, you have to pay for 2 items.
Anyway, never make the trolley full when you leave your home, especially if you are not bringing souvenirs to your family or friend. Most of the travelers buy something at the destination and brings back to their homes. Therefore I suggest to keep the trolley up to 70% of the capacity when you leave your home. Then, bring a textile made shopping bag with you in case of overloaded situation.