You wiill be given a tax code at the job to reflect the fact that you earn profits from the shop - this would normally be a basic rate code, or a 40% code if your profits from the shop take you into higher rate tax. When you complete your tax return you will show both sources of income,, the tax deducted from the employer, and any refund can then be claimed or underpayment paid over with the rest of your tax bill.
As regards NI, you must continue paying your Class 2 unless your profits from the shop are likely to be below �4635 per year.
You may defer payment of your Class 4 NICs is you think you would be paying over the maximum at the main rate for the year (1% contributions will still be payable and will be collected at the end of the tax year). You can apply on this form
http://www.hmrc.gov.uk/forms/ca72b-06.pdf
(that is last year's form, I couldn't find this year.s but it gives you an idea of what to look at)