With a Website Payment Pro merchant account where PayPal standard is offered as a secondary payment option, can one-click upsells be used with PayPal account payments? If not, what will happen in those cases where customers elect to pay via paypal and we have multiple upsells/downsells in the form of a tree?

If a client chooses to use PayPal Standard as their payment option, it will not be one-click. If the customer selects credit card, it will be.

So in case they use their PayPal account, AND there is multiple upsells/downsells, would they be required to do a PayPal checkout at EACH step? Or just once after the first purchase and then once more at the end of the funnel?

For a PayPal standard payment method, each step is handled as a separate transaction to reduce shopping card abandonment. So yes, they would be required to do the PayPal checkout at each step of the upsell/downsell funnel.

Additional keywords: WPP, checkout, website payment pro