How can I add a customer-selected gift wrapping option?

This tutorial will walk you through adding a "Gift wrap" checkbox to the product page, which when selected will add the product to the cart with the additional £1 charge associated with it being gift wrapped.

First, define some settings to control the behaviour of gift wrapping:

\Aero\Common\Facades\Settings::group('gift-wrap', function (\Aero\Common\Settings\SettingGroup $group) {
    $group->float('price')->default(100)->map(function ($value) {
        return new \Aero\Cart\CartPrice(request()->store()->currency, $value ?? 0);
    });
});

\Aero\Catalog\Models\Variant::settings(function (\Aero\Common\Settings\ModelSettingGroup $group) {
    $group->boolean('has_gift_wrap')->default(false);
});

Now, add a macro for has_gift_wrap so that we can cast it to js, so that it will be available in Vue:

\Aero\Catalog\Models\Variant::macro('getHasGiftWrapAttribute', function () {
    return $this->setting('has_gift_wrap');
});

Aero\Catalog\Models\Variant::addCastToJs('has_gift_wrap');

We can now display a checkbox on the product.twig page when a variant supports it:

<div v-if="variant && variant.has_gift_wrap">
    <input type="checkbox" id="gift-wrap">
    <label for="gift-wrap">Gift Wrap? ({{ setting('gift-wrap.price') }})</label>
</div>

And then we listen to an item being added to the cart and set the has_gift_wrap bool from this checkbox:

{% js %}
    <script>
        window.AeroEvents.on('product.add-to-cart', (data) => {
            const el = document.getElementById('gift-wrap');

            data.has_gift_wrap = el?.checked ?? false;

            return data;
        });
    </script>
{% endjs %}

Finally, we extend the cart item builder to add the gift wrap option when selected:

\Aero\Store\Pipelines\CartItemBuilder::extend(function (\Aero\Cart\CartItem $item, \Closure $next) {
    $hasGiftWrap = $item->model->setting('has_gift_wrap') && request()->get('has_gift_wrap');

    if ($hasGiftWrap) {
        $price = setting('gift-wrap.price');

        $option = \Aero\Cart\CartItemOption::create('Gift Wrap');

        $option->setPriceInc($price->inc()->value);

        $item->addOption($option)->setNotCombinable();
    }

    return $next($item);
});

Checkout the aerocargo/product-options module, for an easier way to do things like this.