getRate.ts

getRate() {
  this.items$ = this.service.getConversion()
    .pipe(
      tap((data) => {this.conversionRate = data.rates.USD;}),
      switchMap(() => this.createItemsObservable())
    )
}

private createItemsObservable(): Observable<Item[]>{
 return this.itemsQuery.selectAll()
  .pipe(
      map(itmes => itmes.filter(item=> {return !item.received;})),
      map(res => res.sort((item1, item2) => 
        { return (item1.deliveryDate > item2.deliveryDate ? 1 : -1) }))    
  );
}
  
// then you can also make your ngOnInit shorter
ngOnInit() {
    this.items$ = this.createItemsObservable();
    const source = interval(10000);
    this.subscription = source.subscribe(val => this.getRate());
}