Add to Cart Button

The Add to cart button is normally generated automatically by HikaShop. You don't need to add it yourself.
If you don't see it, it's either that you activated the "catalogue mode" option of the configuration or that your product is free and the option "display add to cart button for free products" of the configuration is turned off, or that your product has a quantity of 0.

If you can't edit the product description, it's probably because of your HTML editor. In the configuration of HikaShop, you have the option "editor" where you can select another editor.

HikaShop displays the add to cart button with the default look like on your second image. However, it uses the CSS class "button" so that it adapts automatically to your template's CSS. If you see the button in blue, that's because your template CSS defined the button class like that. So if you want it to display normally, you should either:
change your template CSS to not change the button display style.
change the html of the button so that it doesn't use the button CSS class.

For option one, I can't help you as it depends on your template.
For option two, you can do that with a template override on the button display:
www.hikashop.com/support/documentation/6...tation.html#override
That way you won't loose your modifications. But if you're not a developer, that will be hard. So instead, you can just edit the file administrator/components/com_hikashop/helpers/cart.php and change the line:
$html= '<input type="submit" class="button hikashop_cart_input_button" name="'.$map.'" value="'.$name.'" '.$ajax.$options.'/>';

to:
$html= '<input type="submit" class="hikashop_cart_input_button" name="'.$map.'" value="'.$name.'" '.$ajax.$options.'/>';

Mmm. Displaying "Add to cart" instead of "Submit" is just a matter of changing the name of the button in the view.
Now if you want to have both a product in the cart and the data of the form submitted at the same time it's just not possible. Maybe your form component allows you to redirect the user somewhere when the form is submitted (I think that some form components have such option) in which case you could just set the add to cart URL there so that the data would be submitted and then the user would be redirected to the add to cart URL.

After the checkout, your POST data will be lost unless you save it somewhere, in the session or in the database. Also, most of the time, the payment notification is done with a server to server notification so the data in the session would be different since it wouldn't be the session of the user but the session of the payment server.
Anyway, to do stuff when the order is confirmed, the best is to create a hikashop plugin and implement the event onAfterUpdateOrder as explained in the documentation. You can look at the history plugin's code for an example.

I don't see why I wouldn't be possible to create such plugin.

What you can already do is to add custom item fields of the table "item" so that they appear on the product pages. That way you can create complete forms on your product pages.
Then, you can always rearrange the show file of the view product if you need to customize the display of the product page. You could come up with a product page which would like like a ticket submission form for example without too much trouble.
What's nice with that is that the data is handled automatically by HikaShop, add in the order and in the emails. Also, you can even change the price based on the data entered by the user by creating a quantity price calculation override plugin as explained in the documentation.
So for example, you could have the user enter some text to be translated and have him to pay $1 per word in the text he entered. You can go quite far with that system without doing much coding.

I don't know about such thing.

However, if you click on the export button in the sales listing, it will output a CSV file which will contain all the orders information along with the products of the order and the custom item fields of these products.
It could be adapted to output it in xml or json if you wanted. But if you wanted to have it as a webservice you will need to build a component or a plugin and base yourself on that export.

Thank you for the feedback.
I didn't know about that extension. I'm sure it could help others !

So you would have to export the table jos_hikashop_order_product with that extension to extract the custom item fields entries.