This means exactly what Customer Relations wrote to you. If you want an electronic certificate then they will grant you one for $150. Of course this is much lower than what they initially offered (i.e. $250). This is why you were advised to simply accept the $250 voucher already offered to you. THEN request this $250 e-cert. to be coverted into a $250 paper voucher that you can use systemwide. You will need to write to the approp. address that you are entitled to use depending on your status on UA. If you don't have any status then e-mail to customer relations by going to the following website:
http://faq.ua2go.com/display/4n/kb/a...=1&r=0.4170549

If you can't access the url, google "customer relations united airlines". You are welcome to type your letter, but of course you may also use your pen and snail mail them. Address you can get online too.

Good luck!