Hi,
If you want to have a price for that labor appearing separately from the product, then then only way I can see is that you create several products "labor" in an unpublished category and add them to the other products as "option".
You can for example have one for a 5 min labor, one for a 10 min labor etc.
You'll also want to remove the "no" choice in the option dropdown by removing the line:
$option_values[] = JHTML::_('select.option', 0, JText::_('HIKASHOP_NO'));
from the file "option" via the menu Display>Views.
Then, supposing that the weight of all your products is 0, you could set a weight of 1 kg for the 5 min labor, 2kg for the 10 min labor, etc.
And then, you could set a minimum weight restriction of 18kg in your shipping methods so that they would only display on the checkout if there is at least 90min of labor products in the cart.
Then, make sure that you still have the "shipping" view in your HikaShop checkout workflow, and the system won't allow the checkout to finish if there is no shipping method selected and you'll be good to go.
As a side note you can also use dimensions and volume restrictions if you're already using the weight of the products to restrict the shipping methods.