flat shipment rate for multiple of 6?

Hi all, it's nice to become part of this comunity, this is my first web site using opencart and I didn't find any solutions anywhere here, I apologies in case this issue was already addressed.

I was trying to find a solution for a website I am developing, basically I was wondering if there is already a solution that doesn't allow the user to check out less he buys 6 or modules of 6 items.

to be more clear my customer does sell wine and he can ship only in box containing 6 or 12 bottles, he want to sell using a flat shipment rate

so the customer will pay a flat rate that is the same for 6 or 12 bottle but and become the double for 18 or 24 items and so on...
e.g.
flat rate = $5
6 bottles or 12 = 5$
18 bottles or 24 = $10
30 bottles or 36 = $ 15
and so on...

the problem is that the customer must be forced to order 6 or modules of 6.

does anyone know an easy way or is it there any extension that would solve this problem without pain?

thank you in advance
S

If you wanted to use the built-in Weight Based Shipping, you could set the weight of each bottle to 1, and then create rates like this:

5:, 6:5.00, 11:, 12:5.00, 17:, 18:10.00, etc.

This would cause the "no shipping methods" error message to be displayed during checkout if the quantity of 6 wasn't met, so the customer wouldn't be able to check out. You can change the text of this message in /catalog/language/english/checkout/checkout.php.

If you want something with a little more flexibility, you could get Quantity-Based Shipping for this. You could then set up rates like this:

0-5 = -9999
6-6 = 5
7-11 = -9999
12-12 = 5
etc.

Thank you Johnathan, very quick and very kind, I will try you solutions and let you know.
Have a grat day
S