Which grading company are you planning to use? There are a few options. AFA are the best known and (with a high grade) will add the most value. But as they're USA based, it's a long and expensive process.
UKG are UK based so a bit cheaper (not necessarily quicker) you can pay most grading companies more for a faster service, which is one of the 'extras'.
Before deciding to grade you should look over the condition carefully, as any score lower than an 80 can actually reduce the value. Collectors of graded items are generally only interested in high grades, so if the box has any major creases or tears then you might not make your money back on the grading fee.
It's worth noting that to an 'outsider' grading scores may seem incredibly harsh, it's not like a percentage grade. A tiny mark can reduce the grade significantly, if you look on eBay you will find AFA 60 or 70 items that looks pretty good!
Other extras may include a certificate or extra condition report, I wouldn't bother with those for your boxed Jawa, they are more relevant to things like prototypes.