Problem with Paypal “Add to Cart” buttons

I am creating a website for a non-profit organization that has Paypal Payments Standard. (The site is currently in preview mode – domain name not pointed there yet). I have created two Paypal “Add to Cart” buttons for their membership page – each button has a dropdown box with price option, another dropdown box, a couple of text boxes. Add the code to the page in the text field. When testing the buttons, they go straight to the Paypal login (home) page instead of directing to the payment page. I don’t really want to use a plugin, because none of the ones I’ve researched allow all the options I need. I’ve tried clearing my cache before testing, tried testing on different devices, browsers, all without success. Oddly, once in a great while, the button will direct to the right place. Then go right back to leading to the Paypal home page again. What am I doing wrong?

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