The ISHPSSB uses the PayPal payment gateway to process card payments, because of its relatively low cost as well as broad support for worldwide used bank cards. This gateway also allows you (or, repeatedly hints you) to use a PayPal account, if you have it; and if not, to possibly make an account. But there is no need to have a PayPal account, you can “Check out as guest” and not create an account with them.
To pay with your own card and not use PayPal, you have to first click Checkout (either the prominent PayPal checkout, or the less-prominent “Pay without a PayPal account” option; see image) — both buttons lead to the same webapp, titled “Log in to PayPal”. At the bottom of this pop-up, choose the less-prominent “Pay by Debit or Credit Card”. (You can ignore the assumption it makes for the country; on the image here the UK flag, a guess based on browser info.)
Once you have chosen to pay by card, you get a form to fill in all the needed details. Here you can choose the correct country, if needed. It also requires a phone number and email address to contact you in case of problem (and/or to have all the data to make you a PayPal account).
Before you confirm payment at bottom: If you did not want a PayPal account, make sure you are not inadvertently creating one! There is a slider “Save information and create a PayPal account” which should be off by default (light grey, see image). In that case the confirmation button says “Pay now as guest” or similar.
If you have chosen to create a PayPal account, the confirmation button says “Create Account & Pay Now”, as well as ask you to create a password, give your nationality, and your date of birth.