When you have the vouche on your screnn hit the "Prt Scr" key. You'll not see anything happen at this stage but if you then open a new document in MS Word or whatever word processing program you use and then press Ctrl + V (together) the "screenshot" that you've just taken will appear. Then print the page in the usual way.
Some companies like this offer a coupon printer package that you download so you can print their coupons easily- it may be worth checking the groupon site
There is nothing wrong generally with groupon and the like, but some companies have been overwhelmed by having offered large discounts and then many resulting orders they are struggling to fulfill.
I usually do all my xmas shopping online. This year though there are as many bargains to be found on the high street.....and I can haggle too!