PS4 shipping

Hello there, Do you have any tutorials on how to set up the custom shipping costs. I am really having a lot of trouble understanding how it all works. I am able to calculate the shipping costs for individual items easily but i don't know what to do if someone wants to add multiple items to the shopping cart (I know the shipping cost price myself) but i don't know how to submit/enter it to the shipping settings. For example i am selling cards, i have been able to calculate shipping costs for 1 card and then for 2 cards but it won't let me add for 3 cards. each card weighs approx 0.1kg so i have added the weight up to 0.2kg and then to 0.3kg for 3 cards (the postage for 3 cards remains the same as 1 card ie £1.00.
Also i sell prints if someone wants to buy one print it ships at £3.50 if someone also wanted to purchase a card with the print it would still ship at £3.50. So what do i put in the boxes in the shipping settings to make this work.

Thank you (I hope this makes sense) A video tutorial on this would be very helpful