Since self-employment taxes start when you’ve earned $400 or more of self-employment income (doesn’t include W-2 wages), the IRS wants you to report that income as soon as the gross income is at or above $400.
If your clothing purchase satisfies the requirements found in the IRS Publication 17, then they can be deducted as a business expense. Your clothing must be specifically required by your employer and must not be suitable for taking the place of your regular clothing. If your clothing does not satisfy these requirements, consider them a personal purchase.
Yes, since a gift certificate is essentially a promise to do work or produce a product in the future, it’s considered a liability (similar to a debt owed) on your financial statements. Once you perform the service or hand over the merchandise they purchased with the gift certificate, you take that amount out of the liability bucket, and move it into your revenue bucket.