As it's a SP you are wallpapering for I would suggest you give her a tempting quote she finds hard to resist then, once you're into the job, you add on a few extras. Any cracks in the wall will require crack infill mastic, so that could be;
,
CIM + £20.
When the job is finished, you will have to spend overtime washing your overalls, so;
OWO. + £30
And, of course, you are a professional so should also charge for your design and fitting knowledge;
DFK. + £50
If she complains just shrug your shoulders, give her a big smile and say "Sorry, ees esstra, bby".