I'm just trying to figure out if I absolutely NEED to file as a foreign entity in South Carolian due to the nature of my business.
Doing business in the state is broadly construed. The state statute lists those things that are NOT considered to be doing business in the state. If you do much of anything other than the things listed there, you are doing business in the state and need to register as a foreign business entity there if the LLC is organized in Delaware; That list of things thare are not considered doing business in the state is found in SC statute section 33-44-1003, which states:
SECTION 33-44-1003. Activities not constituting transacting business.
(a) activities of a foreign limited liability company that are not considered transacting business in this State within the meaning of this article include:
(1) maintaining, defending, or settling an action or proceeding;
(2) holding meetings of its members or managers or carrying on other activity concerning its internal affairs;
(3) maintaining bank accounts;
(4) maintaining offices or agencies for the transfer, exchange, and registration of the foreign company's own securities or maintaining trustees or depositories with respect to those securities;
(5) selling through independent contractors;
(6) soliciting or obtaining orders, by mail or through employees or agents or otherwise, if the orders require acceptance outside this State before they become contracts;
(7) creating or acquiring indebtedness, mortgages, or security interests in real or personal property;
(8) securing or collecting debts or enforcing mortgages or other security interests in property securing the debts, and holding, protecting, and maintaining property so acquired;
(9) conducting an isolated transaction that is completed within thirty days and is not one in the course of similar transactions of a like manner;
(10) transacting business in interstate commerce; and
(11) owning, without more, an interest in a limited liability company organized or transacting business in this State.
(b) For purposes of this article, the ownership in this State of income-producing real property or tangible personal property, other than property excluded under subsection (a), constitutes transacting business in this State.
(c) This section does not apply in determining the contacts or activities that may subject a foreign limited liability company to service of process, taxation, or regulation under any other law of this State.
I strongly advise you to get advice from a SC business attorney and tax attorney to get this right. The fees you pay for good advice will save you a lot more money and wasted effort later on should you get this wrong by doing it yourself.