I've checked all three enclaves and I don't see the shoulder style page under what Urgarlag sells. Same has been for a friend, but it seems others have been able to buy it.
The team has a fix in progress for this. However, it won't be included in Update 30 and instead will go out with the first incremental later in June. Because of this, the arms pack availability will be extended in the Crown Store to July 7th, giving you more time to purchase the shoulder page after the fix goes out. Thank you for your patience here!
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