You don't. Charging a surcharge is strictly prohibited by most payment gateways. PayPal for example will terminate your account if discovered you are doing this.

www.paypal.com/us/webapps/mpp/ua/useragreement-full#no-surchages

In short you can charge a handling fee, but it must be applied to all gateways equally. Also CBSubs does not have the functionality to add an additional fee for using a specific gateway. My recommendation is to simply increase the price of your plans by half the cost of the transaction fees.

Additionally transaction fees are a percentage of the total charge. So even if you added that to the plans price it would increase the total charge, which would increase the transaction fee. It's an infinite cycle of trying to recoup the transaction fee.

In the UK the law is different to the USA - UK merchants can charge surcharges based on payment method and indeed the Government allows this - there are many UK websites that charge fees for PayPal payments and I even reported one to PayPal who told me directly in a Resolution Centre case that UK merchants are allowed to do this. See:

That's good to know. I was not aware the UK merchants were permitted to charge a surcharge.

Its a bit discouraging that you would tell your users what they can and cannot do - by the way its not only PayPal - we were planning on charging for an internal standing order payment (offline) which is an intensely manual and laborious process and where we have to recoup costs.

I was simply informing you of my understanding of gateways policies regarding surcharge. I also informed you the limitations of CBSubs. CBSubs does not have functionality to alter the price of a plan based off payment gateway. It's something we've planned and would like to implement, but at this time does not exist. This will likely just be a new parameter added to Promotions to allow for increasing and decreasing the price based off gateway.

Is there a way that you can suggest using php script or something like that that can add the "admin fee" or "surcharge" or whatever it is called? It doesn't make sense to create 12 plans for 4 items that could have any on eof three payment methods...

I do not have a workaround to suggest. I assume the UK laws regarding this require disclosing the surcharge so it'd need to be a sub total line on the basket like Promotions or Tax, which is more complicated than just increasing the price.