dropPrefix()

dropPrefix.swift

// this particular API/implementation for demonstration only,
// not necessarily quite what will be proposed
extension Collection where SubSequence == Self {
    /// Drop n elements from the front of `self` in-place,
    /// returning the dropped prefix.
    mutating func dropPrefix(_ n: IndexDistance) -> SubSequence {
        // nature of error handling/swallowing/trapping/optional
        // returning here TBD...
        let newStart = index(startIndex, offsetBy: n)
        defer { self = self[newStart..<endIndex] }
        return self[startIndex..<newStart]
    }
}
// soon...
extension String: Collection { }

Given this, here’s your example code written using it (compacted a little for brevity):

struct Product {
    var id, group, name, description, currency: String
    var inStock, ordered, price: Int
    
    var priceFormatted: String {
        let whole = (price/100)
        let cents = price - (whole * 100)
        return currency + " \(whole).\(cents)"
    }
    
    init(inputrecord: String) {
        // note, no copying will occur here, as String is
        // copy-on-write and there’s no writing happening
        var record = inputrecord
        
        id          =     record.dropPrefix(10)
        group       =     record.dropPrefix(4)
        name        =     record.dropPrefix(16)
        description =     record.dropPrefix(30)
        inStock     = Int(record.dropPrefix(10))!
        ordered     = Int(record.dropPrefix(10))!
        price       = Int(record.dropPrefix(10))!
        currency    =     record.dropPrefix(1)
    }
}
let record = "123A.534.CMCU3Arduino Due     Arm 32-bit Micro controller.  000000034100000005680000002250$"
let product = Product(inputrecord: record)
print("=== Product data for the item with ID: \(product.id) ====")
print("group          : \(product.group)")
print("name           : \(product.name)")
print("description    : \(product.description)")
print("items in stock : \(product.inStock)")
print("items ordered  : \(product.ordered)")
print("price per item : \(product.priceFormatted)")
print("=========================================================“)