Optional service multiplying by tire quantity instead of charging once per vehic

Hello,

We have a tire shop configured with HikaShop where customers can purchase tires and additional services. Most services are charged per tire unit (mounting, nitrogen inflation, etc.), but we have one specific service that should be charged once per vehicle, regardless of tire quantity: "Steering alignment/geometry for passenger cars".

Current problem: This service is configured as a product characteristic, and when a customer adds 2 tires to cart and selects this optional service, the system multiplies it by 2 (85.95 € × 2 = 171.90 €), when it should only charge 85.95 € once, as this service is performed on the vehicle as a whole, not per individual tire.

What we need:

This service should remain optional (not mandatory)
It should be visible always, even when buying a single tire
When selected, it should add only 1 unit to cart, no matter how many tires are being purchased
All other services should continue working per tire unit as they do now

Question: How should we configure this service to charge it once per vehicle instead of multiplying by tire quantity? Should we use a different configuration method (option instead of characteristic, or separate product)?

Please see attached screenshots:

Product frontend view showing the service
Cart showing the service multiplied by 2
Backend product configuration

Thank you for your assistance.

Hello,

I'm using the Price Calculations plugin to add an optional alignment service to tire products. The formula uses a division operation to charge the service once per order (not per tire), but this is causing tax calculation inconsistencies.

We are using Hikashop: 6.1.1

Plugin formula: {price} + ({alignment_service} / {cart_product_quantity})

Custom field configuration:

Table: item
Column: alignment_service
Values: 0, 42.975, 72.00 (without tax)
Tax: 21% Spanish VAT
"Show prices with tax" = Yes

The problem:

With 2 tires + alignment (value 42.975€):

If field value = 42.975€ (without tax):

Product page: 445.40€ (❌ missing ~4.51€ tax on alignment)
Should show: 449.91€
Checkout total: 899.82€ ✓ (correct)
Checkout unit price: 475.67€ (❌ should be 449.91€)

If field value = 52.01€ (with tax):

Product page: 449.91€ ✓ (correct)
Checkout total: 910.74€ (❌ tax applied twice, +10.92€)
Should be: 899.82€

Question:

How should I configure the custom field value (with or without tax) when using division in the formula? The division by quantity seems to cause tax calculation issues.

Expected behavior:

With 2 tires + alignment (one axle) - field value 42.975€ (no tax):
Product page: 449.91€ (with tax)
Calculation: (350.34 + 21.49) × 1.21 = 449.91€
Checkout unit price: 449.91€ (with tax)
Checkout total: 899.82€
Calculation: 449.91 × 2 = 899.82€

Attached files:

plugin.png: Price Calculations plugin configuration
field.png: Custom field "alignment_service" configuration
imagen1.png: Product page, field value 42.975€ (without tax)
imagen2.png: Checkout, field value 42.975€ (without tax)
imagen3.png: Product page, field value 52.01€ (with tax)
imagen4.png: Checkout, field value 52.01€ (with tax)

Could you please advise on the correct configuration for this scenario?

Thank you for your support.

The change you suggested doesn't seem to affect the cart calculations at all. The product page calculations remain correct, but the cart hasn't changed from the previous version. Let me explain the issues I'm detecting in the cart:

The amount of 481,13 € comes from the following operation (all prices with VAT included):

481,13 € - 398,15 € (tire price) = 82,98 €
82,98 € - 47,90 € (mounting × 2) - 3,62 € (eco-tax × 2) = 31,46 €
31,46 € - 26,00 € (alignment / 2) = 5,46 €

These extra 5,46 € come from adding the alignment VAT twice: (52 × 21 / 100) / 2 = 5,46 €

So the alignment VAT is being duplicated.

Additionally, the prices marked with red circles (images in attachments) are very confusing not only for the user but also for us.

I would like to know if the product list in the cart table could be improved so that the unit price shown is for a single tire with its individual options, i.e., tire price + eco-tax + mounting, which in this case would be 423,91 €, and below it a line for alignment (a unit cost since it's per vehicle) of 52 €.

Ideally, the cart should look like this:

"Tire Title" + tire price (398,15 €) ... Unit price 423,91 € ... Qty 2 ... Total 847,82 €

Alignment/wheel geometry: Yes, one axle (+ 52,00 €) ... Unit price 52,00 € ... Qty 1 ... Total 52,00 €

─────────────────────────────────────────────────────────────── 

Total ............................ 899,82 €

VAT .............................. 156,17 €

Amount without VAT ............... 743,65 €

I'm attaching screenshots of the current cart along with screenshots of the plugin configuration and the custom field.

Thank you for your support.

Hello,

I've just tested the new version and I confirm that the VAT issue is completely resolved. The calculations are now correct both on the product page and in the cart.
The prices are calculated perfectly:

Unit price: 449,91 €
Total (2 tires): 899,82 €
The alignment VAT is no longer duplicated

Excellent work, the plugin works exactly as I needed.
Now I would like to ask you about the cart display I mentioned in my previous message. Would it be possible to display the options (Eco-tax, Mounting, Alignment) as independent lines in the cart table instead of as text below the product name?
Can this be configured in Hikashop or would it require modifying the cart template?
Thank you again for your help and for the quick solution.