Short answer. Not on the terms of the Labor Code article people quote, because it was repealed. Republic Act No. 10361 (2013), the Batas Kasambahay, now governs domestic employment, including work outside the household, and its rules on assignment and wages are the ones that apply.
What the law says
No househelper shall be assigned to work in a commercial, industrial or agricultural enterprise at a wage or salary rate lower than that provided for agricultural or non-agricultural workers as prescribed herein.
Labor Code, Article 145 — Assignment To Non-Household Work. Read the full provision →
The Code provision no longer governs
Article 145 of the Labor Code provided that No househelper shall be assigned to work in a commercial, industrial or agricultural enterprise at a wage or salary rate lower than that provided for agricultural or non-agricultural workers as prescribed herein. That article was expressly repealed by Republic Act No. 10361 (2013), the Batas Kasambahay, whose repealing clause struck out the entire Labor Code chapter on the employment of househelpers. The words are kept on this site as history. Whether a kasambahay may be sent to the family store, and on what pay, is now answered out of that Act rather than from the article above.
The principle to hold on to
The idea behind the old article is the one to keep in mind, even though the article is gone: a household cannot use the domestic wage to buy commercial labour. Minding the counter, stocking shelves or serving customers is work for the business, and a business pays business wages. Republic Act No. 10361 sets out what a domestic worker may be required to do and what she must be paid, so an employer thinking of extending her duties into an enterprise should read the Act's own provisions on assignment and on wages before doing it, not the repealed text.
Why it matters to the household as well
This is not only the worker's problem. If a person hired as a kasambahay is in fact working substantially in a commercial enterprise, the arrangement may not be domestic employment at all, and the household may find itself the employer of an ordinary employee with the full set of statutory obligations that follow — minimum wage for that work, hours and premium pay, and the social contributions attaching to it. Blurring the line is usually more expensive for the employer than paying properly for the store hours in the first place.
Sorting it out before it becomes a dispute
Be concrete: write down what she actually does in the house, what she is being asked to do in the store, and how many hours each takes. Then check that against the duties and wage rules in Republic Act No. 10361, and pay the store hours as what they are. Keep the written contract, the registration papers and a record of payments, and update the contract if the work genuinely changes. This page gives general legal information about which law now applies rather than advice on your own household arrangement.