3 custom fields string

[user_fra]

Hi (Joomla 3.9.24 Virtuemart 3.8.,
I created 3 custom fields string for online course subscriptions, with different time options available (month 1 - month 2 ... quarter 1 and 2 etc ...) but I don't have a single starting price to whom apply prices increases. I need a zero point to start (empty value) and users should be able to select their option (only one in this case). Unfortunatly sometimes people click add to cart without selecting an option. In this case in the cart there is the product at zero euro. Then they come back to the product detail page and select an option, so the order has 2 item products, one at zero euro and the other one with the correct price and option. Other times they complete the order at zero value.
I need to prevent they can add to cart a zero value.

Of course I could create child products, but it is not cnecessary because we are not talking about stockable products, one product is just perfetc.

Is there a way without touching Virtuemart core, to reach what I need?
This is an example in a test site: https://www.taichionline.it/test/corsi-tai-chi-scaricabili/catalogo-completo/test-webinar-detail.html

Thank you, best regards.
Francesco

[pinochico]

You created wrong idea and product...

You need new analyze and create system for products:

Basic price must be for one option from customfield and customfield is only plus price

[user_fra]

Thanks for your kind answer.
Could you please explain how to reach the same goal using another way?
Of course I don't want to create dozens of products, for the same online course.

Thank you, my best regards
Francesco

[pinochico]

http://demo.virtuemart.net/skirt-knock-rock-detail

[user_fra]

Thank you for your kind answer.
Please, following your example which one should be the base price?
Please also consider that if a student clicks on the add to cart button, he will find a product with the base price.

You can see the real values

Monthly payments option
Month 1 = € 70
Month 2 = € 70
...
Month 9 = € 70
(The name of the option is important even if the price is the same)

Quarter recovery option
Quarter 1 and 2 = € 312
Quarter 1, 2 and = 3 € 390

Quarterly payments option
Quarter 1 = € 150
Quarter 2 = € 150
Quarter 2 = € 150
(The name of the option is important even if the price is the same)

Wouldn't it be clearer and simpler to have a back-end option that says don't add the product to the cart if "this specific product" has value equal zero?
In any case, I'm ready to find a different solution using Virtuemart, that's why I'm writing in this useful forum.

Kind regards
Francesco