Hello,
there are a few notes on this plugin.
The referral system is not working well. The same users register using their links to receive bonuses, there is little sense from this.
It is better to get a referral bonus only after the referral replenishes his account.

I also want to display the information of your plugin not automatically, but using a link, how can I do this?
This is implemented in your banner ad plugin, such as:
<?php if(function_exists('osp_banner_button')) { osp_banner_button({banner_id}); } ?>

Yes it would make sense to redesign referal system, not sure what you mean by your second question.

I want to display links for paying for premium status ads by pasting the code into the theme files.
For example, will this work on the item page?
<?php osp_show_itempay_promote($item) ?>
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Here on this page
domen.com/osclasspay/cart
need to make it possible to remove ads from the cart to pay for premium status if the user has selected several ads by accident

@buninsan
There is such button, just your theme does not show it.