How to build a build-a-box in WooCommerce
Any six chocolates, any four candles, a mixed case of wine, bought as one order. Four steps, no code.

Set it up
You need BundleBoss Pro. The build-a-box is one of its bundle types.
In the BossMage menu, click Create a bundle, then choose Build-your-own box.

Set How many can they pick, then add the products they choose from.
Pick Different sizes instead to sell packs at one price each, such as a box of 6 and a box of 12.

On Review, name the box, choose a layout and set the price.
The price can be a percentage off, an amount off, or one fixed price for the whole box.

Click Publish bundle.

The box is now live. Your customer fills it, watches the count and the total, and buys it as one order. To change anything later, edit the bundle like any product.
Common questions
Can WooCommerce do a build-a-box without a plugin?
No. Core WooCommerce cannot let a customer pick a set number of products into one box and buy them as one item. You need a bundle plugin. The build-a-box is part of BundleBoss Pro.
Can I charge more for a bigger box?
Yes. Choose Different sizes in step 2 and give each size its own price, or add bulk discounts so the price per item falls as the box grows.
Does the stock of each product still go down?
Yes. An order reduces the stock of every product the customer put in the box, and restores it on a refund.
Every setting, with pictures: build-a-box documentation. Or try the cheese hamper, with priced box sizes.