Agree with Tom Elmore. In resolving this, think one needs to remain aware of the distinction between formal written orders issued and field communications conveyed (either orally or via note) for carrying out formal orders.
Believe there is no known record of any written order by Lee to Longstreet for the July 3 attack. Following the day's operations on July 2, Lee merely wrote in his post-battle report ('OR': Vol. XXVII, Part II, at p. 320) , …"Longstreet, re-enforced by Pickett's three brigades, which arrived near the battlefield during the afternoon of the 2d., was ordered to attack the next morning,…"…
It appears that Lee's attack order was communicated verbally.
Found some disparity in the commentary, however, about who might have verbally delivered the order.
In 'Gettysburg', Stephen Sears wrote (at pp 346-47). …"Lee's orders for July 3 reached Longstreet probably around 10 o'clock that Thursday night. The orders themselves are not on record – they were probably delivered verbally."….
More specifically, in 'Lee's Lieutenants', Douglas Freeman indicates that during the night of July 2-3, Lee rode up to Longstreet's field headquarters and delivered the order personally.
There is no mention (or implication) in any of these references, that any formal written order for the charge was issued.
This subject makes me also wonder whether there are any instances during the war of Confederate Corps commanders ever insisting on receiving a formal written order before carrying out a disputed verbal one. (Can vaguely recall at least one instance of a Southern Division commander in the western theater requiring a written order from high command, before joining in a launched attack).