Hello,
Paypal classical cannot work in a popup, they are specific restrictions made in Paypal website to avoid that.
Now, some API in Paypal can authorize a popup, like "paypal adaptive" which handle special js function to embed a popup within the website. And maybe API such as "Paypal REST" (or Paypal Advanced in Germany) can also provide that feature.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.